Various improvements to the test suite.

- Rename RUN-TESTS to RUN and export it from BABEL-TESTS.
- New test CODE-CHAR-NILNESS checks whether we can encode all
  code points below BABEL:UNICODE-CHAR-CODE-LIMIT.
- TEST-UNICODE-ROUNDTRIP: don't try to encode non-characters.
- UTF8-ILLEGAL-CODE-POSITIONS: new test adapted from MGK's UTF8
  test suite.
- Disable a few tests for CCL: UTF-8B.* and CODE-CHAR-NILNESS.