Failing test for broken ANSI escape handling in LinePrefixOutputStream

[lihaoyi]

If the ANSI escape is split over multiple writes, the current color tracking fails

[lihaoyi]

The issue seems to be that flush() calls writeOutBuffer(), so a single line may be written more than once if broken into multiple writes with flushes in between, and if the ANSI escape code is broken over the multiple writes then the fansi.Str.apply( call does not properly recognize the partial escape codes on each side of the flush