tests/config: added a TAP-based test environment for modules/configs

I moved the test files to module directories because it allows
vendoring of whole modules including tests etc.

The test environment provides convenience functions and produces
test output in TAP format. Ideally all tests should use a common
format, so that CI can parse it provide better test output on PRs.
It seems like Gitlab CI doesn't support anything yet, but there
are two sort-of standards supported in CI tools - TAP and JUnit.
I chose TAP because it's easier to read for humans, cmocka supports it,
and it should be easier to adapt Deckard. There are also tools to
convert TAP into JUnit XML file.

Also added more tests for global functions and variables, and the
test tool now also tracks coverage (if `luacov` is installed).