test-lib.sh: Suppress the "passed all ..." message if no tests run

test-lib.sh: Suppress the "passed all ..." message if no tests run

If a test script issues a test_done without executing any tests, for
example when using the 'skip_all' facility, the output looks something
like this:

    $ ./t9159-git-svn-no-parent-mergeinfo.sh
    # passed all 0 test(s)
    1..0 # SKIP skipping git svn tests, svn not found
    $

The "passed all 0 test(s)" comment line, while correct, looks a little
strange. Add a check to suppress this message if no tests have actually
been run.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>