tests: vcs: use pytest fixtures instead of xUnit-style setup_method/setup_class

A subsequent commit will introduce parametrization of fixtures to avoid each
test class to be duplicated for every supported VCS explicitly.

But this concept does not mix with xUnit-style methods setup_class and
setup_method, because the latter are called much earlier than pytest
fixtures and thus cannot use variables set only later by a fixture.

Therefore, use a real fixture to set up the class and test methods.