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>