Make tests represent realistic flows

[AdrienHorgnies]

• The git commit graph didn't correspond to what one would observe with a real release flow; for example, some tests initialize the branch develop with a version corresponding to a test branch.
• The tests assumed the HEAD would finish on the branch develop after the plugin invocation. A test cannot assume the test subject succeeded its job, as its very purpose is to test that.
• The test cases missed the case where the version of the branch develop increases after a feature branch is created and before the feature branch is merged. And it so happens the plugin behavior in such a case is erroneous.

Notes on the PR:

• I only handled integration tests for feature-finish as this is the goal I initially worked on for #367. Meanwhile, my client stopped using this plugin. If you like these changes, I invite you to replicate them on the other integration tests.
• The test 4 fails; I believe the plugin is wrong. #367 would fix it.
• I noticed my fork was behind, I'll come back to this PR to make sure GitHub flows reflect the behavior I observed locally.

[AdrienHorgnies]

Out of the three tests I modified, I broke two of them. They pass locally but fail on the CI. I didn't have time to debug.

[AdrienHorgnies]

@aleksandr-m Sorry, but I won't have time to finish any of the PR I started. My client moved away from gitflow-maven-plugin, and work is piling up, so I can't afford to allocate time to any of the PR I filled.

I opened:

• 367

• 368

• 379

If you liked any what's in the PR, feel free to fork / copy them, as I'll delete my fork in 2 weeks.