Move initialization of GoogleMock and flags to main from test_main_lib

Bug: None
Change-Id: Ie3aed382d4e468c4adbfdbcc1bdb3f069d3eaae2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181364
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31909}

test/test_main_lib.h

@@ -25,6 +25,8 @@ class TestMain {
   // Initializes test environment. Clients can add their own initialization
   // steps after call to this method and before running tests.
   // Returns 0 if initialization was successful and non 0 otherwise.
+  virtual int Init() = 0;
+  // Temporary for backward compatibility
   virtual int Init(int* argc, char* argv[]) = 0;
 
   // Runs test end return result error code. 0 - no errors.