Severity: wishlist

There's a lot of repetitive code to get at the resource directories for
tests.  That's obviously boring, so I'd propose something like the
following two macros to make it easier:

(ert-resource-directory)
(ert-resource-file NAME)

Please see the attached patch, the second of which uses the macros in
our test suite.  Not sure if they should be in ert.el, so I put them in
ert-x.el for now to play it safe.

Any comments?