tests: make test_must_fail fail on missing commands

The point of it is to run a command that produces failure. A
missing command is more likely an error in the test script
(e.g., using 'test_must_fail "command with arguments"', or
relying on a missing command).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -598,6 +598,9 @@ test_must_fail () {
 	elif test $exit_code -gt 129 -a $exit_code -le 192; then
 		echo >&2 "test_must_fail: died by signal: $*"
 		return 1
+	elif test $exit_code = 127; then
+		echo >&2 "test_must_fail: command not found: $*"
+		return 1
 	fi
 	return 0
 }