Test references without 'END' comment

[nikitinas]

Usage example:

doc.md

<!---test1-->
<!---test2-->
<!---test3-->

Expected korro behavior: if <!---END--> comment is not found before the next test reference (or end of file), current test reference is interpreted as empty (equivalent to <!---END--> at the line after test reference)

[devcrocod]

Expected behavior could result in content being removed between the two directives.
This issue was marked as a bug because no exception was thrown if there was no closing directive.