test-lib.sh: Add check for invalid use of 'skip_all' facility

The 'skip_all' facility cannot be used after one or more tests
have been executed using (for example) 'test_expect_success'.
To do so results in invalid TAP output, which leads to 'prove'
complaining of "Parse errors: No plan found in TAP output".

Add a check for such invalid usage and abort the test with an
error message to alert the test author.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -383,6 +383,10 @@ test_done () {
 	case "$test_failure" in
 	0)
 		# Maybe print SKIP message
+		if test -n "$skip_all" && test $test_count -gt 0
+		then
+			error "Can't use skip_all after running some tests"
+		fi
 		[ -z "$skip_all" ] || skip_all=" # SKIP $skip_all"
 
 		if test $test_external_has_tap -eq 0