message: tests: fix admission control filter test
  Test added by uses RequestHeaderMapImpl constructor made private
  in
diff: @@ -149,7 +149,7 @@ success_criteria:
  EXPECT_CALL(controller_, requestSuccessCount()).Times(0);
  // We expect no rejections.
  - Http::RequestHeaderMapImpl request_headers;
  + Http::TestRequestHeaderMapImpl request_headers;
  EXPECT_EQ(Http::FilterHeadersStatus::Continue, filter_->decodeHeaders(request_headers, true));
  }
  