Test was still failing because it checked the document content before closing.
This commit forces document flush before this check, making the test successful.
CRs always appear at the same modulus 2 offset in test strings, thus bug
https://github.com/mate-desktop/pluma/issues/100 is never detected.
Setting substring lengths to 3 causes the test to detect bug #100, thus failing,
as it should yet do.
Clement Lefebvre
Patrick Monnerat
infirit
Stefano Karapetsas
Perberos