selftest: only use Options.options.SELFTEST_PREFIX if it's not the default

selftest: only use Options.options.SELFTEST_PREFIX if it's not the default

The --with-selftest-prefix option is typically specified as argument
to './configure' ! Overwriting it for 'waf testonly' should only
happen with an explicit 'waf testonly --with-selftest-prefix=/some/path'.

This fixes a regression introduced by
commit edc1cedf932de2bc068da9a5db844ac4c48f4324
(selftest: Actually honor --with-selftest-prefix).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>