Add NIO test utilities

[scottmarchant]

Summary

Adds shared test utilities to simplify async runtime assertions. Umbrella: #2.

Details

• d635942 feat: Add testing utilities that make a number of nio-related tests much easier to run.

Usage Examples

• Capture thrown errors while keeping a default value:

let value: Int = try assertNoThrowWithValue(try maybeThrowing(), defaultValue: 0)

• Assert a condition becomes true within a deadline:

assert(flag.load(ordering: .relaxed), within: .milliseconds(50))

Tests exercising these helpers: #6 and #10.