- Removed @testing.supported. Dialects in development or maintained outside

- Removed @testing.supported.  Dialects in development or maintained outside
  the tree can now run the full suite of tests out of the box.
- Migrated most @supported to @fails_on, @fails_on_everything_but, or (last
  resort) @unsupported.  @fails_on revealed a slew of bogus test skippage,
  which was corrected.
- Added @fails_on_everything_but.  Yes, the first usage *was*
  "fails_on_everything_but('postgres')".  How did you guess!
- Migrated @supported in dialect/* to the new test-class attribute __only_on__.
- Test classes can also have __unsupported_on__ and __excluded_on__.