* testcover-mark-line - Mark whole line with Testcover

I developed this for my another package in development (you can't find
it on the internet), which has more than 6000 lines of code and more
than 50 tests trying to cover all non-UI parts of it.  Finding the
testcover marks in such a large file is extremely is hard for me, so I
wrote it to mark whole line also, with another face.