[pmb-status 2/4] use SPDX license header
Depends on and contains patch from !1876 (merged). I got fed up with the boilerplate in the files, so I did a little refactoring.
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.