Move googletest inclusion to third-party (#11207)

### Summary
TSIA

### Test plan
CI

Author: jathu

CMakeLists.txt

@@ -143,6 +143,11 @@ else()
   set(CMAKE_CXX_FLAGS_RELEASE "-O2 ${CMAKE_CXX_FLAGS_RELEASE}")
 endif()
 
+if(EXECUTORCH_BUILD_TESTS)
+  set(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR ON)
+  include(CTest)
+endif()
+
 add_subdirectory(third-party)
 
 if(EXECUTORCH_BUILD_EXTENSION_TRAINING)
@@ -254,11 +259,6 @@ if(EXECUTORCH_BUILD_PTHREADPOOL)
   )
 endif()
 
-if(EXECUTORCH_BUILD_TESTS)
-  set(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR ON)
-  include(CTest)
-endif()
-
 # TODO(dbort): Fix these warnings and remove this flag.
 set(_common_compile_options -Wno-deprecated-declarations -fPIC)
 
@@ -472,11 +472,6 @@ install(
 )
 install(FILES tools/cmake/executorch-config.cmake DESTINATION lib/cmake/ExecuTorch)
 
-# Add googletest if any test targets should be built
-if(BUILD_TESTING)
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/third-party/googletest)
-endif()
-
 if(EXECUTORCH_BUILD_ARM_BAREMETAL)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/backends/arm)
 endif()

third-party/CMakeLists.txt

@@ -10,6 +10,10 @@ if(EXECUTORCH_BUILD_PYBIND)
   add_subdirectory(pybind11)
 endif()
 
+if(BUILD_TESTING)
+  add_subdirectory(googletest)
+endif()
+
 # MARK: - flatbuffers
 
 if(WIN32)