Add tests for MavenInvoker

[sridamul]

Added a test plugin to test the Maven Invoker functions.
If there is better way to test MavenInvoker fn, let me know. Thanks!

Testing done

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[jonesbusy]

I wouldn't call maven directly from such unit tests

What about mocking Invoker invoker = new DefaultInvoker() perhaps moving it to a private member instead of local variable ?

Even if it's private it can be mocked (See https://github.com/jenkinsci/plugin-modernizer-tool/blob/main/plugin-modernizer-core/src/test/java/io/jenkins/tools/pluginmodernizer/core/github/GHServiceTest.java#L72)

So the invoker.execute(request); can be mocked and asserting the correct request (maven are are passed)

[jonesbusy]

Try mocking the Invoker class (move it as a member rather than local variable)

[jonesbusy]

Not sure if planning to work on this, but since we added Guice unit testing the MavenInvoker is probably more easy now