fix warnings: syncShutdownGracefully not available in async contexts

[weissi]

Motivation:

Warnings are annoying. Companion of #2994

Modifications:

Fix all the syncShutdownGracefully not being available in async contexts warnings.

Result:

Everybody happier.

[weissi]

there was actually a bug here which would always leak the group...

[Lukasa]

Thanks! ✨

[Lukasa]

/__w/swift-nio/swift-nio/Tests/NIOPosixTests/SerialExecutorTests.swift:59:13: error: initialization of immutable value 'eventLoopGroup' was never used; consider replacing with assignment to '_' or removing it
        let eventLoopGroup = self.group
        ~~~~^~~~~~~~~~~~~~

[weissi]

/__w/swift-nio/swift-nio/Tests/NIOPosixTests/SerialExecutorTests.swift:59:13: error: initialization of immutable value 'eventLoopGroup' was never used; consider replacing with assignment to '_' or removing it
        let eventLoopGroup = self.group
        ~~~~^~~~~~~~~~~~~~

ta, fixed