When running unit tests, some may have side effects which cause a
subsequent test to break. This can sometimes be seen when using 'ut dm'
or similar.

Add a new argument which allows a particular (failing) test to be run
immediately after a certain number of tests have run. This allows the
test causing the failure to be determined.

Update the documentation also.

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