Fix test/stdlib/VarArgs.swift on the minimal/standalone bot (#39029)

* Fix test/stdlib/VarArgs.swift on the minimal/standalone bot

* Update VarArgs.swift

Author: kubamracek

test/stdlib/VarArgs.swift

@@ -3,6 +3,13 @@
 
 import Swift
 
+// Work around the inability for static-library based Swift runtime builds to
+// directly link against Darwin.swiftmodule by using a benign dependency on
+// StdlibUnittest.
+// https://bugs.swift.org/browse/SR-15118
+import StdlibUnittest
+runAllTests()
+
 #if canImport(Darwin)
   import Darwin
   #if _runtime(_ObjC)