tests: Skip tests if required tools are not available

There is no reason to fail a test if the required testing tool is not
present on the system, so skip the test instead of failing it.

Signed-off-by: Leo Sandoval <lsandova@redhat.com>
Reviewed-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

tests/ext234_test.in

@@ -12,17 +12,17 @@ fi
 
 if ! which mkfs.ext2 >/dev/null 2>&1; then
    echo "mkfs.ext2 not installed; cannot test ext2."
-   exit 99
+   exit 77
 fi
 
 if ! which mkfs.ext3 >/dev/null 2>&1; then
    echo "mkfs.ext3 not installed; cannot test ext3."
-   exit 99
+   exit 77
 fi
 
 if ! which mkfs.ext4 >/dev/null 2>&1; then
    echo "mkfs.ext4 not installed; cannot test ext4."
-   exit 99
+   exit 77
 fi
 
 "@builddir@/grub-fs-tester" ext2_old