textiter: fix bug in case insensitive backward search

textiter: fix bug in case insensitive backward search

'win.lines' contains the same content as the GtkTextBuffer, so to find
@match_start, forward_chars_with_skipping() is called with
skip_decomp=FALSE (the last parameter). So far so good.

On the other hand, the content 'lines' (the needle split in lines) is
casefolded and normalized for a case insensitive search. So,
forward_chars_with_skipping(..., skip_decomp=TRUE) must be called only
for the portion of text containing the needle.

Since 'start_tmp' contains the location at the start of the match, we
can simply begin at that location to find the end of the match.

Unit tests are added.

https://bugzilla.gnome.org/show_bug.cgi?id=758698