Add missing RUN line

Thanks to @bob-wilson for this fix

Author: Dave Abrahams

test/Prototypes/Algorithms.swift

@@ -9,6 +9,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//
+// RUN: %empty-directory(%t)
 // RUN: %target-build-swift -g -Onone -DUSE_STDLIBUNITTEST %s -o %t/a.out
 // RUN: %target-run %t/a.out
 // REQUIRES: executable_test