Summary of changes:
- Fix the test resources introduced by #783 by moving the `regex`
  fields, such that the test framework does not skip them with a "Not a
  valid test case" message.
- Revert the changes introduced by #815, as those are simply incorrect.
- Extend the test coverage introduced by #815 by (a) updating the test
  regexes to match their intended semantics and (b) include a few
  negative test cases.
- Partially revert the change introduced by #783: the use of
  `Matcher#find()` is correct, but the `hasStartAnchor` and
  `hasEndAnchor` logic introduces more bugs than the issue it aims to
  solve.
- Extend the test coverage introduced by #783, by introducing regexes
  that are not covered by the `hasStartAnchor`/`hasEndAnchor` logic.
- Update the Joni regular expression integration such that it passes
  more of the test cases.
- Disable the "trailing newline" test cases, as these are currently not
  handled correctly by either regex implementation.