test: Update tests to use the skip feature

Some tests currently return 0 when they want to be skipped. Update them to
return -EAGAIN instead, so they are counted as skipped.

A few tests are in two parts, with the latter part being skipped in
certain situations. Split these into two and use the correct condition for
the second part.

Signed-off-by: Simon Glass <sjg@chromium.org>
5 files changed