test: vboot: Parameterise the test

This test is actually made up of five separate tests. Split them out so
that they appear as separate tests.

Unfortunately this restarts U-Boot multiple times which adds about a
second to the already-long vboot test, about 8 seconds total on my
machine. We could add a special 'teardown' test afterwards but if the
tests are executed out of order that would not work.

Changing test_vboot into a class causes it not to be discovered and makes
it different from all other tests.

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