[CodeCompletion] Move a test to long test suite

rdar://problem/48698892

Author: rintaro

validation-test/IDE/slow_fixed/rdar45511835.swift renamed to validation-test/IDE/slow/rdar45511835.swift

@@ -1,6 +1,9 @@
 // RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename=%s | %FileCheck %s
 
+// REQUIRES: long_test
+
 // This used to take ~6 min to complete.
+// Now it's 2 min: rdar://problem/48818341
 
 func testing() {
   return (["a"] + [1].map { String($0) })