Migrate fixture tests

[Muscraft]

Compared to source code based tests, the benefits of using toml to specify tests haven't felt worth it to me recently. That, combined with the fixture tests mostly being about the colored output, made me realize that we should probably get rid of the toml fixtures. So I went ahead and did just that. I am happy with the changes, with the only caveat being that the test names all have ::case at the end.

test ann_eof::case ... ok
test ann_insertion::case ... ok
test ann_removed_nl::case ... ok
test ann_multiline2::case ... ok
test ensure_emoji_highlight_width::case ... ok
test ann_multiline::case ... ok
test fold_bad_origin_line::case ... ok
test issue_9::case ... ok
test fold_leading::case ... ok
test fold_ann_multiline::case ... ok
test fold_trailing::case ... ok
test strip_line_char::case ... ok
test strip_line::case ... ok
test multiple_annotations::case ... ok
test simple::case ... ok
test strip_line_non_ws::case ... ok