Don't install embedded copy of GTest

Goole Test itself documents the `INSTALL_GTEST` option as something that projects embedding it should set to `OFF`. Leaving it on means that projects downstream from libraries that embed it are likely to encounter conflicting copies. This is particularly annoying because GTest does not maintain anything like a stable API, and so causes builds to fail if include paths pick up an inappropriately installed copy ahead of the one that the code wanted.

Fixes #488

test/CMakeLists.txt

@@ -2,6 +2,7 @@ find_package(Threads REQUIRED)
 
 set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 set(BUILD_MOCK ON CACHE BOOL "" FORCE)
+set(INSTALL_GTEST OFF CACHE BOOL "" FORCE)
 set(CMAKE_POLICY_DEFAULT_CMP0048 NEW)
 
 if(YAML_USE_SYSTEM_GTEST)