Add unit tests for Searcher and Reader

Even though we will be removing the wrappers soon, the test coverage
should be complete and we could simply remove these files later.

test/meson.build

@@ -9,6 +9,8 @@ tests = [
     'book',
     'manager',
     'opds_catalog',
+    'reader',
+    'searcher'
 ]
 
 if build_machine.system() != 'windows'