RFC: Formalize stress tests

Hello,

I'd like to formalize the stress tests in the test suite by annotating them like long_test. Can anybody think of a good reason not to do this? It is a straightforward and simple change:

https://github.com/apple/swift/pull/15211

Here is the rational from the pull request:

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.

Dave