capture_unittests should support --test_data_path option to specify a test data file's path.

Current implementation fixes it to "DIR_SOURCE_ROOT/media/test/data/".
https://cs.chromium.org/chromium/src/media/base/test_data_util.cc?l=179&rcl=f1ef4405b08f701d2bdae9494e0cdf92506de98e

We want to specify the path by a command line parameter like jpeg_decode_accelerator:
https://cs.chromium.org/chromium/src/media/gpu/jpeg_decode_accelerator_unittest.cc?l=874&rcl=2f8f18b0a61db49c199f52d58123226b778fa843