[coverage] tokenizer.c: coverage of interactive pieces

[mdboom]

tokenizer.c seems to have no coverage for a few functions related to interactive usage, e.g. tok_underflow_interactive and tok_concatenate_interactive_newline.

I'm not sure how to assess the value of these things -- except they do seem to have a lot of pretty complex logic.

[mdboom]

Tracking as part of #94808 instead.