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.

tests/meson.build

@@ -127,3 +127,14 @@ foreach layout : [
 endforeach
 
 endif
+
+if get_option('find_orphans')
+    test('test_find_orphans',
+        cargo_script,
+        args: ['run'] + cargo_build_flags
+            + ['--example', 'find_orphan_layouts',
+                '--', meson.source_root() + '/data/keyboards/'],
+        timeout: timeout,
+        workdir: meson.build_root(),
+    )
+endif