Use grep for matching args with sh

Fixes the following shellcheck error:

    SC2081 (error): [ .. ] can't match globs. Use a case statement.

https://www.shellcheck.net/wiki/SC2081

test/scripts/rpm-ostree_mock

@@ -5,7 +5,7 @@
 # Write invocation with arguments to a file to allow making assertion.
 echo "$*" >> /rpm-ostree_calls
 
-if [ "$*" != *"--append"* ]
+if ! echo "$*" | grep -q "\--append"
 then
     # Caller is trying to read kernel arguments.
     cat /proc/cmdline