Add testing/corpus to .gitignore

TBR=halyavin@chromium.org

Review URL: https://codereview.chromium.org/1032853004
diff --git a/.gitignore b/.gitignore
index 623bf8d..529e888 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 /build/Release
 /build/gyp
 /out
+/testing/corpus
 /testing/gmock
 /testing/gtest
 /v8