Summary:
When Gtest cannot be discovered by CMake config file, the tests fails to compile:
```
In file included from /home/docker/opensource/folly/folly/test/common/TestMain.cpp:23:
/home/docker/opensource/folly/folly/portability/GTest.h:32:10: fatal error: gtest/gtest.h: No such file or directory
   32 | #include <gtest/gtest.h>
      |          ^~~~~~~~~~~~~~~
```
This PR fixes this problem with a classic `find_package`.
Pull Request resolved: https://github.com/facebook/folly/pull/1331

Differential Revision: D20323404

Pulled By: yfeldblum

fbshipit-source-id: 2e41f99172c504063c00c45a9fa5c08a26bcc0c8