Brown paper bag: don't use `grep -v`

Brown paper bag: don't use `grep -v`

For some inexplicable reason I'd gotten the semantics of `grep -v` totally
backwards, thinking it somehow would make the exitcode from grep indicate
the non-match of the specified pattern.

This fixes the broken tests and gives me a valuable lession about shell
programming at the same time.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>