Make python tests run independent of cwd

This will allow us to run all .TestCase test suites independent of a users current path.

eg. this will work now:

.../fdroidserver$ tests/common.TestCase

thus save us a lot of annoying cd-action while doing test driven development :)

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This MR also fixes common.ensure_final_value which may implicitly return None where the corresponding aapt output parser would return '' instead.