For some reason the final plugin test TestSlashURLNomrlaizer seems to be having and the last CI job times out after >3 hours! I suppose we should set a timeout configuration for GitHub CI as well as Jenkins. I'll look into that.
I'll investigate the hanging/timeout issue ASAP.

OK I found an issue. I'm not sure if it is the sole issue but it needs fixed.
Several plugin tests are failing with

```java.lang.NoClassDefFoundError`
...
Caused by: java.lang.ClassNotFoundException
...

Clearly this is a classpath issue. 
JUnit 3 & 4 relied on the `<junit>` ant task which scanned Java source files (`.java`).
JUnit 5 scans for compiled class files. I suspect this is the issue.
I'm working on a solution and will update ASAP.

It's not TestSlashURLNormalizer but one of the HTTP protocol tests which is hanging:

   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(java.base@11.0.28/Native Method)
        at org.apache.nutch.protocol.AbstractHttpProtocolPluginTest.launchServer(AbstractHttpProtocolPluginT
est.java:208)
        at org.apache.nutch.protocol.AbstractHttpProtocolPluginTest.launchServer(AbstractHttpProtocolPluginT
est.java:227)
        at org.apache.nutch.protocol.httpclient.TestProtocolHttpClient.testNoPreemptiveAuth(TestProtocolHttp
Client.java:112)

Other HTTP protocol tests are failing.

... and the reason is: AbstractHttpProtocolPluginTest (in core, o.a.n.protocol) is already upgraded to JUnit 5 while the HTTP protocol plugin test classes aren't yet, although they inherit from the abstract test class.

Simplest solution: revert the changes to AbstractHttpProtocolPluginTest until the plugin tests are migrated.

Update on PR

@sebastian-nagel thanks for the comments 👍
I came to the same realization this evening when I was able to pick this issue back up and diagnose further. I just pushed a fairly substantial update which upgrades all plugin unit tests as well as those in core.
I kept the changes in plugin to a minimum, following the same process as for core

1. identify all JUnit 3 test classes and API usage and upgrade to JUnit 4
2. Upgrade all JUnit 4 API usage to JUnit 5
3. Usage static imports for all Assertions

Future work considerations

1. I haven't made use of @Tag for any unit tests. We could add this later down the line. See 2.10. Tagging and Filtering.
2. I noticed that unit tests output is now written to a logs directory which may or may not be desirable. Was this always the case? If we want to change this behavior we can adjust the directory that the forked JVM initializes in.
3. JUnit 4 is still pulled in as a transitive dependency, I believe of MRUnit. I logged https://issues.apache.org/jira/browse/NUTCH-3125 to address this and basically also to get rid of MRUnit if possible.

Let me apologize for the change in plan. I realize this PR it a beast. This was not my intention. I spent too much time working on the hanging Ant process(es) due to complications between JUnit 5 in core and JUnit 3 & 4 in plugin. I decided to not let that get in the way of the bigger picture.