Fix FoundationTest build on macOS

[kgominyuk]

FoundationTest build on macOS has been broken since #2912.
Compilation fails with error cannot convert value of type 'SwiftError' to type 'NSError' in coercion.
I guess we have to exclude related tests on Darwin platform, but leave them in DarwinCompatibilityTests.

[MaxDesiatov]

@swift-ci please test

[Slayer741]

Not only Foundationtest;
I ran into a little issue while compiling LLVM+LLDB for mac/ios

i was building on MacOs Big Sur 11.0 (20A5343)
with Xcode 12.3 Beta

} API_AVAILABLE(ios(11.0)) API_UNAVAILABLE(macos, watchos, tvos) NS_SWIFT_NAME(URLSessionConfiguration.MultipathServiceType);
                                                                 ^
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.3.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSURLSession.h:657:66: note: please report this issue to the owners of 'Foundation'
} API_AVAILABLE(ios(11.0)) API_UNAVAILABLE(macos, watchos, tvos) NS_SWIFT_NAME(URLSessionConfiguration.MultipathServiceType);
                                                                 ^
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.3.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSURLSession.h:811:3: warning: imported declaration 'NSURLSessionDelayedRequestDisposition' could not be mapped to 'URLSession.DelayedRequestDisposition'
} NS_SWIFT_NAME(URLSession.DelayedRequestDisposition) API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0));
  ^
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.3.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSURLSession.h:811:3: note: please report this issue to the owners of 'Foundation'
} NS_SWIFT_NAME(URLSession.DelayedRequestDisposition) API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0));

[lxbndr]

@Slayer741 Hmm, this looks unrelated to the scope of this patch. We're focused on breaking changes introduced in #2912. It is correct that TestFoundation has other issues when built with latest Xcode. I think they should be addressed separately.