[SR-3615] Corelibs XCTest does not re-export Foundation

[briancroom]

Previous ID	SR-3615
Radar	rdar://problem/29929469
Original Reporter	@briancroom
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	1
Component/s	XCTest
Labels	Bug
Assignee	@briancroom
Priority	Medium

md5: 6a7740aee2b80266d4aa43519adad590

Issue Description:

An internal engineer has reported that he keeps stubbing toes on a frustrating inconsistency between macOS and Linux:
= On macOS, you can use Foundation data types (such as `Data`) in an XCTest unit test without `import Foundation`
= On Linux, using Foundation data types in unit tests does require `import Foundation`

To provide consistency, Corelibs XCTest should re-export Foundation.

[briancroom]

PR: #183