Add gtest to executorch third-party (#253)

Summary:
Pull Request resolved: #253

Added googletest `v1.11`

Reviewed By: cccclai

Differential Revision: D49106426

fbshipit-source-id: 91e5cf5bf1fa3ee09bd8134a0a960a74b36e69e0

Author: guangy10

.gitmodules

@@ -10,6 +10,9 @@
 [submodule "third-party/gflags"]
 	path = third-party/gflags
 	url = https://github.com/gflags/gflags.git
+[submodule "third-party/googletest"]
+	path = third-party/googletest
+	url = https://github.com/google/googletest.git
 [submodule "backends/xnnpack/third-party/pthreadpool"]
 	path = backends/xnnpack/third-party/pthreadpool
 	url = https://github.com/Maratyszcza/pthreadpool.git