Add missing Foundation imports to fix test build errors on iOS with MemberImportVisibility enabled

[stmontgomery]

This fixes several instances of a build error when attempting to build this package for iOS, or any non-macOS Apple platform. Here's one example

error: instance method 'contains' is not available due to missing import of defining module 'Foundation'
Tests/TestingTests/SwiftPMTests.swift:370:5: note: in expansion of macro 'expect' here
    #expect(testIDs.allSatisfy { $0.contains(".swift:") })

This general kind of build error is being emitted because we adopted SE-0444: Member import visibility by enabling the MemberImportVisibility experimental feature in #1020.

In that PR, I fixed several instances of missing imports, including some for Foundation in test files. But these errors are from usages of String.contains(), and it turns out there are multiple overloads of that function, with an older one in Foundation and a newer one directly in the stdlib Swift module. The latter has newer, iOS 13.0-aligned API availability, and when building our tests for macOS this issue was not noticed previously because SwiftPM artificially raises the deployment target of macOS test targets to match the testing frameworks included in Xcode (when the testing libraries are being used from the installed copy of Xcode).

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[grynspan]

Thanks for fixing this!