Use String.lines in FormattingWriter

I investigated a bit with VisualVM, and it seems that splitting a string
into lines is one of the hot spots. At the moment, String.lines isn’t
optimized, but even if it’s never hand-optimized, the JVM might still
find it marginally easier to optimize several uses of String.lines than
several occurences of String.split('\n'.equals).
Use assertEquals in testCommandLineFiles

Also add the testCase as a message
Remove three uses of String.lines

Replace String.lines.longerThan(1) with String.contains('\n'), which
could be a lot faster (no generic Boolean(Character) predicate).

I did a benchmark where I formatted FormattingWriter.ceylon 1000 times
in a loop (parsing was done outside the loop), and this improved
performance from 56009186211ns to 47157253331ns. I’m not sure how to
convert that into “x percent faster”… but it’s a lot. (Ratio: 0.84)