Fix 2 bugs in multiline find_text()
First bug:
which caused some false positives being returned.
Includes test case for the bug.
See original comment about this bug: https://gitlab.gnome.org/GNOME/evince/-/merge_requests/159#note_1431380
Second bug:
Fix for a bug in double column documents where some
single line matches are wrongly returned as being
multiline matches.
Includes test case for the bug (which needs the test file submitted in test!16 (merged) ).