TSLint test doesn't support multiline failure messages

[gabritto]

TSLint rule tests don't support testing rules that have failure messages that span over multiple lines.
This happens because when TSLint parses a .lint file for the expected failure messages, it always parses lines separately.
For example:

declare var foo;
~~~~~           [This is a multiline
error message]

TSLint will parse the failure message as "This is a multiline" instead of "This is a multiline error message".

ESLint
As far as I could tell, ESLint takes a different approach to rule unit tests, so this issue will be deprecated once #214 is solved.

Workaround
As a workaround, I had my rule take a "single-line" boolean argument. If this is set, the messages the rules generates get joined into a single line. The "single-line" argument is then set in the test TSLint configurations. When running the rule in "production", this boolean is not set, so the messages can preserve their line breaks.