Initial push of CLI command line tests

[JimSuplizio]

1. Update Start.cs to deal with a CommandLineParser issue. If a default verb is set, CommandLineParser doesn't verify whether or not the verb passed in is actually a supported verb. It assumes it to be the default verb. Something like "test-proxy foo" would end up running test-proxy start instead of erroring. The VerifyVerb function checks for this and outputs an error message that would be output by CommandLineParser if there were no default verb and an incorrect verb was passed in.
2. GitStoreIntegrationPushTests.cs - Just updated the description here. These tests changed purpose when we went from branches to Tag. They're now similar tests but acting on different Tags that have different files and different file versions.
3. assets.Tests.Helpers.ps1 - Support functions for the CLI integration tests.
4. CLIIntegration.Tests.ps1 - Pester tests for testing the test-proxy CLI commands. These mirror the functionality in the test-proxy Integration test suite but with a reduced number of tests with the primary goal to test things from the using the CLI. This does require the test-proxy to be installed on the machine and in the path.

[scbedd]

/azp run tools - test-proxy - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[scbedd]

These test scripts are everything I was hoping for. Absolutely stellar tyvm.