Because the amount of features that the simulator is testing is growing,
the test time is taking a long time. This distributes simulator's tests
over three travis machines to decrease test time.
Add options to defined tests that run one-by-one in a sequential fashion
by defining them in $SINGLE_FEATURES, and allow defining features that
should run together by providing a string of ',' separated list of
multiple tests in $MULTI_FEATURES.
Signed-off-by: Fabio Utzig <utzig@apache.org>
Fabio Utzig