tests: Check for orphaned layout files

This adds a test which scans the layouts directory for files which have not been added to the resources file.
The majority os user-submitted layouts don't include layout registration, and I ran out of ideas for how to make it more visible in the documentation. Instead, it's going to be solved on the other side: the CI pipelines shall fail.

To make it easy on experimenters with untidy trees, it's disabled by default.

meson_options.txt

@@ -7,6 +7,10 @@ option('tests',
        type: 'boolean', value: true,
        description: 'Whether to compile unit tests')
 
+option('find_orphans',
+       type: 'boolean', value: false,
+       description: 'Check if all present layout files are included in resources.')
+
 option('newer',
        type: 'boolean', value: false,
        description: 'Build with dependencies newer than those of Byzantium')