fixup: some weaknesses in a self-check test case * t/self-check-cleanup.tap: This test contained several buglets introduced in the recent row of commits that converted the Automake build system to a non-recursive setup. Fix them. Since we are at it, enhance the test to cover also the use of the 'keep_testdirs' environment variable in order to keep the temporary test directories around. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>