Move file utility classes and functions from testing/test_util to
common/file_util, and make them part of libwebm.
Change-Id: If5b25a63b20efacc16b0fecaa8876ade4ecc4b26
Some of the muxer tests have been failing because of incorrect
gold files. Updating the gold files and tweaking the tests so that
they pass in a sensible way.
Change-Id: I7d3dda6cacecda526880d6e7623f9a7678b71265