testutil: switch mock_command to use bash

This change allows use the more advanced features of bash like
array operations (e.g. `${@:2}` to drop the first two arguments
or similar. On fedora/rhel this is a no-op as it is already using
sh -> bash (afaik).

osbuild/testutil/__init__.py

@@ -107,7 +107,7 @@ def mock_command(cmd_name: str, script: str):
         # will break. easy enough to fix by using \0 as the separator but
         # then \n in args is kinda rare
         fake_cmd_content = textwrap.dedent(f"""\
-        #!/bin/sh -e
+        #!/bin/bash -e
 
         for arg in "$@"; do
            echo "$arg" >> {cmd_calllog_path}