build: Add gtest as a submodule

Change-Id: I9a5c440bd81f48afd24961a52595d0ca995cb7bf
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..8a9d107
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "submodules/googletest"]
+	path = submodules/googletest
+	url = https://github.com/google/googletest.git
diff --git a/submodules/googletest b/submodules/googletest
new file mode 160000
index 0000000..9dde7c4
--- /dev/null
+++ b/submodules/googletest
@@ -0,0 +1 @@
+Subproject commit 9dde7c4e8c46a65ad6215440bb396ad219e06ed4