Skip tests based on SKIP_REASON, not return value

Skip tests based on SKIP_REASON, not return value

Currently, {,group_,device_}requires are required to both set
SKIP_REASON and return non-zero. This is somewhat redundant, and will be
inconsistent with skipping from test{,_device} that is about to be
implemented. Let's ignore the return value and always skip the test if
SKIP_REASON is set. The _have_foo and _test_dev_foo helpers should still
return so that they can be chained together.