Extract functionality of test_main into separate library.

Extract functionality of test_main into separate library to be able to
reuse it if another main will be required.

Bug: webrtc:5996
Change-Id: I2925b4240bd0e4fb884b43bb16667ca2d6216bbd
Reviewed-on: https://webrtc-review.googlesource.com/c/105921
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25172}

modules/rtp_rtcp/BUILD.gn

@@ -321,6 +321,7 @@ if (rtc_include_tests) {
       ":rtp_rtcp",
       "../../test:fileutils",
       "../../test:test_main",
+      "//test:test_support",
       "//testing/gtest",
     ]
   }  # test_packet_masks_metrics