Stress tests are, by definition, stressful. They intentionally burn a
lot of resources by using randomness to hopefully surface state machine
bugs. Additionally, many stress tests are multi-threaded these days and
they may attempt to use all of the available CPUs to better uncover
bugs. In isolation, this is not a problem, but the test suite as a whole
assumes that individual tests are single threaded and therefore running
multiple stress tests at once can quickly spiral out of control.
This change formalizes stress tests and then treats them like long
tests, i.e. tested via 'check-swift-all' and otherwise opt-in.
Finally, with this change, the CI build bots might need to change if
they are still only testing 'validation' instead of all of the tests.
I see three options:
1) Run all of the tests. -- There are very few long tests left these
days, and the additional costs seems small relative to the cost of
the whole validation test suite before this change.
2) Continue checking 'validation', now sans stress tests.
3) Check 'validation', *then* the stress tests. If the former doesn't
pass, then there is no point in the latter, and by running the stress
tests separately, they stand a better chance of uncovering bugs and
not overwhelming build bot resources.
