Cleanup of gtest.cc and a couple of unittests

[venik]

Changes:

1. Unused parameter was removed from ::testing::internal::GetCurrentOsStackTraceExceptTop. This code was reworked more than a decade ago aaebfcd.
2. In a couple of tests return type was replaced with const auto&. It's a little bit more convenient because uncouples the implementation of the tests from the return type of ::GetInstance().

make test
Running tests...
...
100% tests passed, 0 tests failed out of 63

Total Test time (real) =   7.67 sec

[venik]

This is a partial split from #3852. It cleans up code a little bit without the change to the ::GetInstance() implementation.