test: Allow optional usage of vendored googletest

Change-Id: I7011edb826b11762f1eb49e8bbc3a19c8006f8c5
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..025c6c9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/subprojects/*/