Two new tests added to prepare TIP 633:

   *   io-75.1: read an invalid utf-8 sequence from a file
   *   io-75.2: write an unrepresentable character to a file

Those tests are in:

   *   TCL 8.6: [2221b9b6e3]
   *   TCL 8.7: [7ab32e9bf1]
   *   TCL 9.0: [67ffff5b7a]

In TCL 8.6 and 8.7, both operation give results and succed. This works currently well.

On TCL 9.0, both operations should fail. Currently, they don't.
In consequence, the TCL 9.0 tests are marked as knownBug.