build: Add a test directory

Add a test directory to host future unit tests.

Change-Id: I5072cb3055386f7b1718edc45241c723d9e63a54
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/.gitignore b/.gitignore
index d1d026c..23155c5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,3 +25,4 @@
 stamp-h1
 phosphor-dbus-monitor
 generated.hpp
+test-driver