Autotest: better AT_TESTED, AT_XFAIL_IF

[Ralf Wildenhues]

It would be cool if Autotest had

- an AT_TESTED that allows non-literals ($LIBTOOL), options, and
  that does *not* exit when the thing is not available,

- a method to mark tests XFAIL only at run time of the test.  Currently,
  the condition of AT_XFAIL_IF is evaluated before the test, which makes
  this so useless.

It would be nice if Autotest had

- hard errors: a way to allow for tests marked XFAIL to exit with FAIL
  if some unexpected condition arises (like the feature request from
  Akim for Automake's TESTS).  A better AT_XFAIL_IF could make this
  less important, but still interesting.

I'd be delighted if someone could implement this.

Thanks,
Ralf