Add --fail-under-line and --fail-under-branch options to return exit status 2/4/6 under a coverage threshold

[mayeut]

This might be useful to fail tests if coverage is not high enough.

[denniswjackson]

Making this --fail-under-lines allows an option for branch coverage as well.

[latk]

I like this enhancement very much. Only two things are missing:

1. Please split the unrelated whitespace changes into a separate commit. Such improvements are good, but not when intermingled with important changes. To do so:

   • switch to your branch: git checkout fail-under
   • soft-reset to the state before this commit: git reset HEAD^. Your workdir is now dirty with the changes from this commit.
   • add the whitespace changes interactively: git add -p --all. You will be asked for each change whether you want to stage it.
   • verify that all whitespace changes are staged and no other changes were staged by running git diff and git diff --staged. To restart the staging, run git reset.
   • commit the staged whitespace changes: git commit
   • commit the actual changes: git commit --all
   • force-push the changes to update this PR: git push --force-with-lease

2. Test cases. You can copy any suitable existing test case and adapt the copy to your needs. We'll have to add three new cases:

   • one where the coverage for both criteria is exceeded. This test case needs no changes other than adding the options to the gcovr invocations in the Makefile.
   • one where the branch coverage fails. Relative to the success-case, we need to raise the coverage limit and then test the exit status after completion: gcovr ... ; test $? = 2 or something like that.
   • one where the line coverage fails. Equivalent changes to the previous case.

Please let me know if you need any help, e.g. with running the test suite.

Related PRs:

• your Use pypy3.5-5.8.0 for testing #175 (merged) fixes the Travis tests. Consider rebasing your changes onto master so that the automated build for this PR succeeds.
• my Fix HTML tests #185 (pending) also affects the tests. I'll have to add the necessary changes for whatever PR is merged second.

[mayeut]

1. addressed by rebasing on master after Fix most PEP 8 warnings #187
2. Done in simple1 txt test

Exit code now depends on which test fails:
2 => line coverage failed
4 => branch coverage failed
6 => both failed