Retrieve test data directory at run time

testing/embedder_test.h

Index: testing/embedder_test.h
diff --git a/testing/embedder_test.h b/testing/embedder_test.h
index 3aa45dfcda03b89fb6f331fdaca2a57c1b9800a9..f68cb9df74e22f6729a3ad8e9979baaaf20be28f 100644
--- a/testing/embedder_test.h
+++ b/testing/embedder_test.h
@@ -80,6 +80,8 @@ class EmbedderTest : public ::testing::Test,
 
   // Open the document specified by |filename|, and create its form fill
   // environment, or return false on failure.
+  // The filename is relative to the test data directory where we store all the
+  // test files.
   virtual bool OpenDocument(const std::string& filename,
                             bool must_linearize = false);