Integration tests can not test assertion behaviors for all of NIO

[gwynne]

Pretty much what the title says. The integration tests can only test the crashing-ness (or not) of assertions in a very small subset of NIO's code (and of course the unit tests can't test it at all). This should be fixed and tests added for such methods as EventLoop.preconditionInEventLoop() and friends.

(Filed by request of @weissi and according to remarks in #1508)

[weissi]

Correct. When I first wrote tests_05_assertions I wanted to save compile time in CI by constructing a reduced NIO package (https://github.com/apple/swift-nio/blob/master/IntegrationTests/tests_05_assertions/defines.sh).

That was a bad idea, we should just take the 30 seconds to compile NIO and build a package that can import NIO instead of trying to cherry-pick just the assertions.

This shouldn't be hard at all really, we need to build a SwiftPM package (much like we're doing now) but we should just add

dependencies: [
    .package("../../..")
]

or so to reach swift-nio. If that doesn't work then we can just use swift package edit --path /path/to/NIO swift-nio like we do in the allocation counter tests.