Issue 9475 - Preserve code layout in unittest body when emitting ddoc.

http://d.puremagic.com/issues/show_bug.cgi?id=9475

**Btw**, the reason the unittest body string is extracted eagerly in `Parser::parseUnitTest` rather than in `emitUnittestComment`, is because by the time `emitUnittestComment` is ran the lexer ends up freeing the memory.