Clean up a few general patterns that are now obviated by canImport
This aligns more generally with the cleanup that the Swift Package
Manager has already done in their automated XCTest-plumbing tool in
apple/swift-package-manager#1826.
Different tests used different os checks for importing Darwin, Glibc and
MSVCRT. This commit use the same pattern for importing those libraries,
in order to avoid the #else branches of the incorrect patterns to be
applied to the wrong platform. This was very normal for Android, which
normally should follow the Linux branches, but sometimes was trying to
import Darwin or not importing anything.
The standarized pattern imports Darwin for macOS, iOS, tvOS and watchOS.
It imports Glibc for Linux, FreeBSD, PS4, Android, Cygwin and Haiku; and
imports MSVCRT for Windows. If a new platform is introduced, the else
branch will report an error, so the new platform can be added to one of
the branches (or maybe add a new specific branch).
In some cases the standard pattern was modified because some test required
it (importing extra modules, or extra type aliases), and in some other
cases some branches were removed because the test will not have used
them (but it is not exhaustive, so there might be some unnecessary
branches).
This should, at least, fix three tests for Android (the three
dynamic_replacement*.swift ones).
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.
[test] Add a timeout to runRaceTest(). Use it to limit test AtomicInt.swift.
This cuts AtomicInt.swift's execution time from several hours to
about ten minutes on slow hardware and slow build configurations.
