Centralise test imports and add os(Android)

[johnno1962]

Hi Apple,

This PR adds os(Android) to the conditional imports and centralises them to reduce some boilerplate.

[parkera]

@swift-ci please test

[spevans]

Is it worth simplifying this to:

#if canImport(Darwin)
    import Darwin
#elseif canImport(Glibc)
    import (Glibc)
#endif

(and also in HTTPServer.swift)

[johnno1962]

The test failure should be fixed now.

[alblue]

There was a failure but it was fixed before I found this out:

13:57:57 
TestFoundation/XDGTestHelper.swift:13:23: error: use of unresolved identifier 'HTTPCookieStorage'
13:57:57         let storage = HTTPCookieStorage.shared
13:57:57                       ^~~~~~~~~~~~~~~~~
13:57:57 TestFoundation/XDGTestHelper.swift:14:26: error: use of undeclared type 'HTTPCookiePropertyKey'
13:57:57         let properties: [HTTPCookiePropertyKey: String] = [
13:57:57                          ^~~~~~~~~~~~~~~~~~~~~
13:57:57 TestFoundation/XDGTestHelper.swift:20:34: error: use of unresolved identifier 'HTTPCookie'
13:57:57         guard let simpleCookie = HTTPCookie(properties: properties) else {
13:57:57                                  ^~~~~~~~~~

[alblue]

@swift-ci please test

[johnno1962]

Sorry @alblue you might need to start the test again. I had to amend the XDG changes so Linux works.

[spevans]

@swift-ci please test

[parkera]

@johnno1962 can you also confirm this works when running the tests in Xcode on Darwin?

[johnno1962]

I wish I could but I’m still running Xcode 9.3 and can’t get past Foundation. Downloading the beta now..

[millenomi]

@johnno1962 Make sure you're using a current Swift compiler toolchain (either built with ninja, or via a swift.org snapshot.)

[millenomi]

@swift-ci please test

[johnno1962]

I’m up to date using master from swift.org now. I’m seeing a seemingly unrelated crash on macOS here: https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/String.subproj/CFStringEncodings.c#L639

[spevans]

@johnno1962 See #1498 for details of that crash.

[johnno1962]

Thanks @spevans It’s a strange crash, a buffer overrun that always crashes at the same place. Anyways, I think this is good to go now. Sorry about the XDG problems. Can I ask, what is XDG?

[millenomi]

@swift-ci please test and merge

[johnno1962]

Thanks @millenomi