[embedded] Disable test that only works in non optimized mode

rdar://165656288

Author: aschwaighofer

test/embedded/existential_cast_fails.swift

@@ -26,6 +26,7 @@
 // RUN:  -wmo -runtime-compatibility-version none %s  -o %t/t4.out
 // RUN: not --crash %t/t4.out 2>&1 | %FileCheck %s --check-prefix=CHECK-T4
 
+// REQUIRES: swift_test_mode_optimize_none
 // REQUIRES: swift_in_compiler
 // REQUIRES: executable_test
 // REQUIRES: optimized_stdlib