I would prefer to have a lot of smaller specific test cases, over chaining a large number of them into a single common input string.  Its hard to figure out what the rules we care about are from a particular assertion when the input is so complex.

Do a lot of little tests for parsing:  testParse_EmptyInput().  testParse_TrailingWhitespace().  testParse_MultipleLinesSamePattern().  testParse_IgnoreComment().  testParse_IgnoreConflictMarkers().  Etc.