tests: use shunit for integration tests

Add a copy of the shunit library (shell, LGPL) to avoid an extra dep from now.
Modify the test setup to use abs dirs in bt-cfg. Rewrite two test suites to use
the shunit lib.
tests: fix bt-cmd-cli tests

Playing one file is enough. Also shorten the duration a bit for quicker test
exec. Finally fix the test by using .wav as a format for auto-guessing.
Previously we used .ogg which is not unique.