Make unit tests run in `--parallel`

With the new object model (see: #1) it should be possible to run unit tests with `--parallel` enabled (because `py` is no longer a global variable). Maybe. Probably…

- `UnicodeDataDoesNotCrashTests. test_iterateAll` should be split by [plane](https://en.wikipedia.org/wiki/Plane_(Unicode)) giving us 17 tests
- `make test` (in `Makefile`) should be updated
