oe-selftest: Allow overriding the build directory used for tests

This may be useful if the parent directory of the original builddir is
not writable, on a lower performance drive, etc.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit dab719e5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>