[Docs] Update googletest docs link

[oToToT]

Summary

The documentation link of Google Test on GitHub have been moved to the top-level docs directory.
Thus, the original link is invalid now.

llvm/docs/TestingGuide.rst

 "test-suite") and are in the ``test-suite`` module in subversion. For
 historical reasons, these tests are also referred to as the "nightly
 tests" in places, which is less ambiguous than "test-suite" and remains
 in use although we run them much more often than nightly.
 
 Unit tests
 ----------
 
-Unit tests are written using `Google Test <https://github.com/google/googletest/blob/master/googletest/docs/primer.md>`_
-and `Google Mock <https://github.com/google/googletest/blob/master/googlemock/docs/for_dummies.md>`_
+Unit tests are written using `Google Test <https://github.com/google/googletest/blob/master/docs/primer.md>`_
+and `Google Mock <https://github.com/google/googletest/blob/master/docs/gmock_for_dummies.md>`_
 and are located in the ``llvm/unittests`` directory.
 In general unit tests are reserved for targeting the support library and other
 generic data structure, we prefer relying on regression tests for testing
 transformations and analysis on the IR.
 
 Regression tests
 ----------------