[test] Fix the query-log-file test
Testing this functionality in a CubicWebTC is awkward because the
wrapping happens in handle_request, which tests basically bypass (they
call core_handle directly, and do their own magic for linking the
request with a cnx). The test method worked when ran on its own, but
not together with the rest of the test class. So use a CubicWebServerTC
instead, which goes through the whole stack.