[test] Require -O stdlib for long-running test (#21769)

Author: milseman

validation-test/stdlib/StringBreadcrumbs.swift

@@ -1,5 +1,5 @@
 // RUN: %target-run-stdlib-swift
-// REQUIRES: executable_test
+// REQUIRES: executable_test,optimized_stdlib
 
 // Some targetted tests for the breadcrumbs path. There is some overlap with
 // UTF16View tests for huge strings, but we want a simpler suite that targets