Don't test connecting to new database if `in_memory_db?`

If `in_memory_db?`, connecting to database on new handler is no schema
and fixture loaded.

Closes #37734.

activerecord/test/cases/query_cache_test.rb

@@ -154,7 +154,7 @@ def test_query_cache_with_multiple_handlers_and_forked_processes
     rd.close
   ensure
     ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
-  end
+  end unless in_memory_db?
 
   def test_query_cache_across_threads
     with_temporary_connection_pool do