Merge branch '2022-09-12-update-pytests-for-more-parellel-support' into next

To quote the author:
This series makes a further attempt to get closer to having all tests
run in parallel. It introduces a new 'make pcheck' option which runs
tests in parallel, skipping those that are not compatible.

A number of fixes are included for existing tests. The vboot test is
updated to only run a single scenario in 'quick' mode.

This makes use of pytest's parallel-testing features. The resulting
times (including incremental building with LTO) on a 16-core machine are
as follows:

   make pcheck        - 1 minute 6 seconds
   make qcheck        - 3 minutes
   make check         - 5 minutes 15 seconds

Note that this is not a fair comparison, since 'make pcheck' omits a
number of tests, even more than 'make qcheck'.