Log capture in tests

This merge request uses Capture::Tiny to log tests. In a multi-threaded application like the test runner, that is superior to the existing method in which no statements can be printed and all information must be sent to the parent process via Test::State.

Going forward, all tests (and also new features) are much easier to debug. The log for each test is available inside that test's working directory in debian/test-out/${suite}/${testname}/ in a file called log.