Fix warnings when building tests on Swift 5.9.2 Linux

[simonjbeaumont]

Motivation:

Since 5.9.2 was released the 5.9 docker tags started resolving to 5.9.2 automatically, which have caused our CI to fail because 5.9.2 includes a new warning, which is promoted to error in our CI.

Explicitly using 5.9.2 tag shows this, but not for 5.9.1:

❯ docker run --rm -v $PWD:/code -w /code -it swift:5.9.1-jammy swift build --build-tests -Xswiftc -warnings-as-errors
Build complete! (24.22s)

❯ docker run --rm -v $PWD:/code -w /code -it swift:5.9.2-jammy swift build --build-tests -Xswiftc -warnings-as-errors
...
/code/Tests/NIOCoreTests/AsyncSequences/NIOAsyncWriterTests.swift:662:9: error: instance method 'wait' is unavailable from asynchronous contexts; Use await fulfillment(of:timeout:enforceOrder:) instead.; this is an error in Swift 6
        wait(for: expectations, timeout: seconds)
        ^
error: fatalError
[464/518] Linking NIOHTTP1Client

Modifications:

Lower compiler guard for test utility backport.

Result:

Works now with both versions:

❯ docker run --rm -v $PWD:/code -w /code -it swift:5.9.1-jammy swift build --build-tests -Xswiftc -warnings-as-errors
...
Build complete! (36.65s)

❯ docker run --rm -v $PWD:/code -w /code -it swift:5.9.2-jammy swift build --build-tests -Xswiftc -warnings-as-errors
...
Build complete! (21.74s)

[weissi]

@swift-nio-bot please test

[weissi]

@swift-nio-bot test this please

[Lukasa]

@swift-server-bot test this please

[simonjbeaumont]

//cc @dnadoba

[Lukasa]

Merging over failing CI, as this is blocking other fixes.