Allow setting a cwd to execute tests from.

Reviewed-by: Dylan Baker <baker.dylan.c@gmail.com>
Build a tests file for the X Test suite.

Build the test suite externally, then drop a symlink from that xtest
top level directory into bin/

This patch was original written by Eric Anholt, but I've cleaned it up
and reworked parts of it before upstreaming, and as a result I've
attached my name as the author so ``git blame'' will send angry mail my
way instead of Eric's.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>