Teletext: Change to text mode should reset the held character

[hoglet67]

Here's another sequence that doesn't quite work correctly:

VDU &91,&61,&9E,&92,&93,&94,&81,&91,&91,10,13

The &81 should reset the held character to a space, and to that as "set after". On JSBeeb the held character remains.

Here's the read hardware:

[mattgodbolt]

With the new teletext tests in place, we can look at fixing this (and locking in the good behaviour to boot!)