Add test for regex matching on multiline docs. Refs #36.

Showcases desired behaviour when matching a pattern using "$regex" on a document field that contains multiple lines of text. (When 's' option is used a dot character should match any character including a new line character '\n'.)

Desired behaviour:
Documents should match pattern like "line1.*line2" when they contain multiple lines of text, say "line1\nline2" and option 's' is used.

Observed behaviour:
No multiline documents match the pattern.

Note that this test does not pass because desired behaviour is not observed.