Disable Dictionary/Set.swift tests that fail in optimize mode

Disable the tests in optimize mode until we have fix to unblock builders.

rdar://27547957

Author: aschwaighofer

validation-test/stdlib/Dictionary.swift

@@ -12,6 +12,9 @@
 // RUN: %line-directive %t/main.swift -- %target-run %t/Dictionary
 // REQUIRES: executable_test
 
+// rdar://27547957
+// XFAIL: swift_test_mode_optimize
+
 #if os(OSX) || os(iOS) || os(tvOS) || os(watchOS)
 import Darwin
 #else

validation-test/stdlib/Set.swift

@@ -12,6 +12,9 @@
 // RUN: %line-directive %t/main.swift -- %target-run %t/Set
 // REQUIRES: executable_test
 
+// rdar://27547957
+// XFAIL: swift_test_mode_optimize
+
 import StdlibUnittest
 import StdlibCollectionUnittest