[lit][googletest] Handle upstream gtest output

Summary:
Upstream googletest prints "Running main() from gtest_main.cc" to stdout prior
to running tests. LLVM removed that print statement in r61540. If a user were
to use lit to run tests that use upstream googletest, however, lit
reports "Running main()" as an invalid test name.

To avoid such a failure, add an extra conditional to `formats/googletest.py`.
Also add tests to demonstrate the modified behavior.

Reviewers: abdulras, ddunbar

Subscribers: ddunbar, llvm-commits, kastiglione

Differential Revision: http://reviews.llvm.org/D18606

llvm-svn: 265034