tests: add regression tests for ul_configs_file_list()

tests: add regression tests for ul_configs_file_list()

Add comprehensive test script for configuration file list functionality
with the following test cases:

- main-etc: Main config file in /etc
- main-usr: Main config file fallback to /usr
- dropin-etc: Drop-in files from /etc
- dropin-usr: Drop-in files from /usr
- combined: Main config + drop-ins from multiple directories
- masking: Same basename in multiple directories (masking behavior)
- no-project: Configuration without project subdirectory

The tests verify proper file discovery, priority ordering across /etc,
/run, and /usr directories, and file masking behavior according to the
Configuration Files Specification.

Signed-off-by: Karel Zak <kzak@redhat.com>