invalid_file_test: add an operator<< for DecodeParam

Improves the --gtest_list_tests output and avoids a valgrind warning in
gtest's testing::internal2::PrintBytesInObjectTo() due to padding in the
structure.

old:
VP9/InvalidFileTest.
  ReturnCode/0  # GetParam() = (0x9d5308, 16-byte object <01-00 00-00 00-00 00-00 37-02 73-00 00-00 00-00>)

new:
VP9/InvalidFileTest.
  ReturnCode/0  # GetParam() = (0x9d5308, threads: 1 file: invalid-vp90-01-v2.webm)

Change-Id: Ifb9c66fba2e72272bd591a3f6273aeb6bda6af4a

test/invalid_file_test.cc

@@ -30,6 +30,10 @@ struct DecodeParam {
   const char *filename;
 };
 
+std::ostream &operator<<(std::ostream &os, const DecodeParam &dp) {
+  return os << "threads: " << dp.threads << " file: " << dp.filename;
+}
+
 class InvalidFileTest
     : public ::libvpx_test::DecoderTest,
       public ::libvpx_test::CodecTestWithParam<DecodeParam> {