Introduce new test harness that reads from test.list files.

- Test harness now recursively reads through directories with test.list files
in them and executes tests based on the information contained within.
- Test.list files currently can contiain further includes of test.list files
in subdirectories or information for tests that can compare the parsed vtt
file to data in a JSON file or, alternatively, can use custom assertions contained
within a js file.