[RemoteMirror] Add missing test.swift file for interop tests.

Author: mikeash

unittests/Reflection/RemoteMirrorInterop/test.swift

@@ -0,0 +1,11 @@
+@_cdecl("test")
+public func test() -> UInt {
+  return unsafeBitCast(c, to: UInt.self)
+}
+
+class C {
+  let x = "123"
+  let y = 456
+}
+
+let c = C()