Move common test flags to a separate library.

This enables the possibility to use these flags from other main
functions.

Bug: b/217701950, b/238157120
Change-Id: I6e9f09c4be39b2ac690c8e6b6da380ef3c81f460
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/269140
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37582}

test/test_flags.h

diff --git a/test/test_flags.h b/test/test_flags.h
new file mode 100644
index 0000000..694b454
--- /dev/null
+++ b/test/test_flags.h
@@ -0,0 +1,22 @@
+/*
+ *  Copyright (c) 2022 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef TEST_TEST_FLAGS_H_
+#define TEST_TEST_FLAGS_H_
+
+#include <string>
+#include <vector>
+
+#include "absl/flags/declare.h"
+
+ABSL_DECLARE_FLAG(std::string, force_fieldtrials);
+ABSL_DECLARE_FLAG(std::vector<std::string>, plot);
+ABSL_DECLARE_FLAG(std::string, isolated_script_test_perf_output);
+
+#endif  // TEST_TEST_FLAGS_H_