The testosbuild.TestCase class creates a fresh store for each test,
because tests should run independent of each other.
This can lead to long waiting times while developing a new test case.
Allow overriding the store used with OSBUILD_TEST_STORE. This should
never be used where tests are actually run. It is a development-only
feature.
Use the unittest module from the standard library. Also, ensure that
separate runs of this test don't share a osbuild store and clean up
after themselves.
With contributions from Ondřej Budai and Tom Gundersen.
