Add optional parameter to gotest.parse_files

We run gotest.parse_files in a 'post' task. If something failed before tests were run, gotest.parse_files errors:

> Command failed: no files found to be parsed

It would be nice to have an optional parameter like it not fail if no files are found, just like 'local_files_include_filter' does for s3.put so that lack of files doesn't look like an error.