[test] Just check that we can /import/ the XCTest overlay (swiftlang#23020)

This would have caught Apple-internal issue rdar://problem/48512910.

Author: jrose-apple

test/stdlib/XCTest-smoke.swift

@@ -0,0 +1,13 @@
+// RUN: %empty-directory(%t)
+// RUN: not test -e %platform-sdk-overlay-dir/XCTest.swiftmodule || %target-build-swift %s -o %t/main
+
+// REQUIRES: objc_interop
+
+import XCTest
+
+// Check that we can actually refer to functions from the XCTest overlay.
+let optionalInt: Int? = nil
+XCTAssertNil(optionalInt)
+
+// Check that the underlying framework was imported.
+_ = XCTestCase.self