test: Allow optional usage of vendored googletest

Change-Id: I7011edb826b11762f1eb49e8bbc3a19c8006f8c5
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.clang-ignore b/.clang-ignore
new file mode 100644
index 0000000..36f17f2
--- /dev/null
+++ b/.clang-ignore
@@ -0,0 +1 @@
+./subprojects