Tests: replace sleep with Thread.sleep(forTimeInterval:)

[compnerd]

sleep is not portable as it is not a standard C function. Use the Foundation function to provide a single codepath across all the targets.

[Kyle-Ye]

On the Windows part this is totally fine.

But for macOS or Linux side, the implementation will change from Darwin.sleep to a Thread.sleep implementation on Foundation.

I am not familiar with the implementation of both and does not know whether it will give us different behavior.

Source: https://github.com/apple/swift-corelibs-foundation/blob/bd2e810a3ff5adf12410666cee74725d94f2dd25/Sources/Foundation/Thread.swift#L121-L156

Should we use a if directive to make sure other platform's behavior is not effected or just use Thread.sleep for all platforms? cc @franklinsch

#if os(Windows)
Thread.sleep(xx)
#else
_ = sleep(xx)
#endif

[compnerd]

Thread.sleep(forTimeInterval:) gives a uniform way to handle sleeping. This is used pretty extensively in the Swift project. There is a Sleep function on Windows that does sleep for the number of seconds.

[daniel-grumberg]

Looks good to me since this is just in testing code.

[compnerd]

@swift-ci please test