Try to guard a test that requires DEBUG().

This is a follow-up patch for r185524. Being assert enabled does not mean
that DEBUG() is enabled, so we need to check the existence of DEBUG() itself.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@185619 91177308-0d34-0410-b5e6-96231b3b80d8