ptest: Allow all test runs to be logged

Check the environment variable `PTEST_LOG_ALL`, and if it is set, write
the output of all test runs to a log file instead of just the failures.
Failed test are still written to failure-nnnn.log, but successful runs
are written to success-nnnn.log.  If the environment variable is not
set, ptest behaves as before, only writing to log files when there is a
failure.

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