Rewrite test runner.

- Use PHPUnit. There are many different ways to use PHPUnit,
  unfortunately, none of them seem to fit quite right with
  the JSON-LD test suite -- without requiring a lot of extra
  and/or duplicated effort. A middle ground was chosen.
Fix typo.
Add link header parse method.
Do not output relative IRIs in RDF datasets.
Handle scalars before objects.
Support more error conditions in expand.
Add term definition error codes, etc.
Add missing semicolon.
Add more error codes, fix error params.
Fix parenthesis.
Fix remote document loading bugs.
Fix typos.
Use remote doc documentUrl as base.
Fix link header parsing bugs.
Use input url not doc; use link header parsed result as an object.
Fix typo.
Improve failed test output; fix typo.
Validate URLs; check for file/json read errors.
Fix typo.
Fix parenthesis.