Add googletest as a submodule.

Change-Id: I8ebd26ce53930b60892bcf4e2378fb587df08da6
Reviewed-on: https://swiftshader-review.googlesource.com/7550
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/.gitmodules b/.gitmodules
index be6b64f..4ec5099 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
 [submodule "third_party/pnacl-subzero"]
 	path = third_party/pnacl-subzero
 	url = https://chromium.googlesource.com/native_client/pnacl-subzero
+[submodule "third_party/googletest"]
+	path = third_party/googletest
+	url = https://github.com/google/googletest.git