[pmb-status 1/4] test/pmb_test: prepare for common test code

Replace the "Add topdir to import path" boilerplate lines in each test file with a simple "import pmb_test". Use the "# noqa" comment if "pmb_test" is not used further in the test file, so flake8 does not complain about an unused module.

Make the path to the testdata available as pmb_test.const.testdata, and use pmb.config.pmb_src to access the topdir in all tests.

This is in preparation for new "pmbootstrap status" related tests, which will have shared test code in test/pmb_test/. Also, this makes the pmbootstrap codebase more consistent with the bpo codebase, which has a similar "import bpo_test" mechanism.