Revise tests for issue JENKINS-70101

[jimklimov]

Context:

A long-standing PR jenkinsci/credentials-plugin#391 aims to fix (Certificate) Credential snapshot() implementation to allow remote build agents to correctly use a Credential (there was originally a mismatch of encrypted bytes and a random KEY specific to each JVM run-time instance to manipulate them).

The original problem was noticed with use of this HTTP Request plugin, so some of the tests in that PR revolved around making sure the multi-node pipelines using this plugin would work correctly.

Reviewers of that PR insisted on not adding dependencies (even test ones) unless strictly required, so this PR is posted to shift the weight of those tests into HTTP Request plugin: it already has dependencies on Credentials Plugin anyway, it does one way or another suffer practical usability issues (even through no fault of its own), and an earlier version of those tests (running only in Jenkins controller's local JVM) was imported and merged years ago as part of PR #120 for a loosely related issue.

The chain of commits in this PR modifies just the HttpRequestStepCredentialsTest file of all the Java code, first to better javadoc its existing data and methods, and later to import code from jenkinsci/credentials-plugin#391 (most of which would be dropped there) as needed to set up the remote build agent and run test cases with pipelines using it. Beside certificate credentials, this also adds testing of username credentials for good measure.

Tests with a remote agent are currently optionally blocked away by static private Boolean credentialsPluginDoesSnapshotsRight which is true if class loader knows about com.cloudbees.plugins.credentials.impl.CertificateCredentialsSnapshotTaker (it is added in jenkinsci/credentials-plugin#391 and so means using a build of Credentials Plugin with the fix for JENKINS-70101 issue) -- if the ultimate fix in that other PR ends up differently identifiable, or later code evolution changes it, this part may need revision. Actually when some fix gets merged over there, a small commit to HTTP Request over here to require a certain minimum version of Credentials plugin and to drop the complexity of this check in the test would also be a decent solution.

• For now, developers tinkering with the test can redefine credentialsPluginTestRemoteAlways = true to do run remote tests and maybe fail trying, to confirm whether the original issue was resolved or not. I thought about configuring it via a JVM property or environment variable, but chose against adding a complex feature into this hopefully temporary fix.

I have verified locally that:

• by default the code committed into this PR branch passes the test suite, skipping the two test cases that should run on remote nodes (credentialsPluginDoesSnapshotsRight is detected as false using the default Credentials Plugin version, developer toggle credentialsPluginTestRemoteAlways is hard-coded as false)
• if I flip credentialsPluginTestRemoteAlways to true, the testCertHttpRequestOnNodeRemote() fails (as expected so far) with

java.io.IOException: Remote call on slave0 failed
hudson.remoting.ProxyException: javax.crypto.BadPaddingException: Given final block not properly padded. Such issues can arise if a bad key is used during decryption.

... but testUsernamePasswordHttpRequestOnNodeRemote() actually passes (is not impacted by botched transfer of SecretBytes)

• I am so far struggling to ensure that the custom build of the fixed Credentials plugin gets used, to test that these HTTP Request tests would pass.

[jimklimov]

UPDATE: Got it to run and pass all defined test cases with custom-built Credentials plugin (which requires a recent ecosystem of Jenkins core and dependency plugins), with auto-detection of the build with fixed JENKINS-70101 working as expected:

• fixed back the default credentialsPluginTestRemoteAlways = false
• modified pom.xml to require at least Jenkins 2.504.3 and BOM 5804.v80587a_38d937
• added https://repo.jenkins-ci.org/incrementals as a new entry in repository and pluginRepository
• added <version>1501.v00f26696ff79</version> to dependency for credentials
• ran mvn clean and began mvn package -U to fetch everything it needs per updated requirements
  ** Might suffice to run the mvn test -Dtest="HttpRequestStepCredentialsTest" (anyhow I regularly do the equivalent in IDEA UI)?

What failed was a too-stone-age approach:

• just modifying Jenkins core version requirement, updating and placing a build of that other PR into target/test-classes/test-dependencies/ (and its dependencies so the minimal versions required by that build of Credentials are satisfied), e.g.:

:; cp ../credentials-plugin/target/jenkins-for-test/WEB-INF/detached-plugins/*.?pi target/test-classes/test-dependencies/
:; cp ../credentials-plugin/target/test-classes/test-dependencies/*.?pi target/test-classes/test-dependencies/
:; wget https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/credentials/1501.v00f26696ff79/credentials-1501.v00f26696ff79.hpi -O target/test-classes/test-dependencies/credentials.hpi

...with this, the run-time unpacked test area seems to be the same as when the version is mentioned in pom.xml and uses files that have the new class in the plugin, but in fact the test Jenkins instance loads something else from somewhere else, it seems.