selftest: do not perform a full build in test_continue
This was fetching and building the toolchain and everything else
against empty download dir and sstate cache, and so was enormously slow.
The test does not need that, it only checks that one fetch task fails and
another succeeds when using bitbake's -k option.
Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
