test: Allow optional usage of vendored googletest

Change-Id: I2646b01eefa954b1afc70ecc04791d53b99d96d7
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