According to saltstack#52001 we need to add the mod_dir to sys.path if we have
any `utils_dirs`. These tests ensure that happens.

When the minion start clean and there are not utils directory in
the extmods cache, any import from a module to any code inside the
utils directory will fail. A second run of the highstate will fix
this issue, as the utils directories are added into sys.path inside
config/__init__.py, as part of the master and minion startup.

This commit add the utils directories during the synchronization of
the custom modules.

Fixes saltstack#51958

In the original PR it assumed paths would be well-formed. This fix uses
pathlib to split out the paths, which should be much more robust, as
well as ensuring that the mod_dir actually does end with the dir in
utils_dirs.