TestFoundation: make test_nullDevice test more portable

[compnerd]

This makes test_nullDevice more portable so that we can run the test
on Windows as well.

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Linux platform

[millenomi]

This needs conflict resolution before I can merge.

[spevans]

Doesnt this need a

#if NS_FOUNDATION_ALLOWS_TESTABLE_IMPORT
    #if canImport(SwiftFoundation) && !DEPLOYMENT_RUNTIME_OBJC
        @testable import SwiftFoundation
    #else
        @testable import Foundation
    #endif
#endif

At the top if you are using NS_FOUNDATION_ALLOWS_TESTABLE_IMPORT ?

[compnerd]

That is already present now. So, this is ready to go as is.

[spevans]

It needs to be in every file that does a @testable import - I just tested this PR on xcode and it wont compile otherwise.

[compnerd]

@spevans - huh? TestFIleHandle already has the @testable import: https://github.com/apple/swift-corelibs-foundation/blob/master/TestFoundation/TestFileHandle.swift#L12-L18

[compnerd]

@swift-ci please test