While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.
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.
Oliver Smith