TAP Directives for XPASS and XFAIL tests with lib/tap-driver.sh

Hello,

I recently was using the tap driver provided by automake, and noticed 
that there currently is no way to declare a test as xfail or xpass apart 
from marking it as "todo". The test ouput however does produce rows for 
XPASS and XFAIL, and the existing TRS format already supports both.

Since the TAP specification allows for custom directives [1], I was 
wondering if there was interest in a patch adding these to 
`lib/tap-driver.sh`. I would add "XPASS" and "XFAIL" directives, and if 
the test did not pass/fail as expected, it would result in an ERROR.

I don't expect this would be too hard to implement and I can have a 
patch soon, but I thought I'd gauge interest first.

Thanks,
Soham