[test] Require optimized stdlib for Collection validation tests

These take too long to run with unoptimized stdlib builds, causing timeout issues.

rdar://problem/46878013

Author: lorentey

validation-test/stdlib/Collection/Inputs/Template.swift.gyb

@@ -70,6 +70,10 @@ def test_methods(test):
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
+// FIXME: the test is too slow when the standard library is not optimized.
+// rdar://problem/46878013
+// REQUIRES: optimized_stdlib
+
 import StdlibUnittest
 import StdlibCollectionUnittest