[Test][stlib] Use conditional grep to enable CHECK-51 assertions

Author: ramonasuncion

test/stdlib/KVOKeyPaths.swift

@@ -1,7 +1,9 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-build-swift %s -o %t/a.out
 // RUN: %target-codesign %t/a.out
-// RUN: %target-run %t/a.out | %FileCheck --check-prefixes=CHECK,CHECK-51,DONT-CHECK %s
+// RUN: %target-run %t/a.out > %t/output.txt
+// RUN: %FileCheck %s < %t/output.txt
+// RUN: grep "check-prefix=CHECK-51" %t/output.txt && %FileCheck %s --check-prefix=CHECK-51 < %t/output.txt
 // REQUIRES: executable_test
 
 // REQUIRES: objc_interop