Suppress file output for the gltestthreading config

Bug: skia:
Change-Id: Icd905ea7e60a05bc3903eb85d111dcf73ce2c4dd
Reviewed-on: https://skia-review.googlesource.com/40690
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 8b6ee0e..c1af4d0 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -344,6 +344,11 @@
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
 
+    const char* fileExtension() const override {
+        // Suppress writing out results from this config - we just want to do our matching test
+        return nullptr;
+    }
+
 private:
     std::unique_ptr<SkExecutor> fExecutor;