sim: Allow slow tests to be skipped

The normal simulation test takes several hours to run on most machines. Allow a
few very slow tests to be skipped by setting the environment variable
`MCUBOOT_SKIP_SLOW_TESTS` to some value. For obvious reasons, this shouldn't be
done if these power failure simulation tests are needed.

With this change, on my desktop Linux machine, the test time with the skipping
goes from about 2 hours, to around 5 minutes.

Signed-off-by: David Brown <david.brown@linaro.org>