It does not always make sense to collect artifacts and data from the
target on failure, e.g. if testing firmware or if the target is not
running an SSH server.
Allow this by setting TESTIMAGE_FAILED_QA_ARTIFACTS to an empty
value.
(From OE-Core rev: 7036f814baa6550e0b1ac2db37fcd51960ee9e48)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>