Fix up the tests I added for string exceptions to be skipped when exceptions are disabled

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@258279 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_append/iterator.pass.cpp

@@ -161,6 +161,7 @@ int main()
          S("12345678901234567890""ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"));
     }
 #endif
+#ifndef TEST_HAS_NO_EXCEPTIONS
 	{ // test iterator operations that throw
     typedef std::string S;
     typedef ThrowingIterator<char> TIter;
@@ -174,4 +175,5 @@ int main()
     test_exceptions(S(), TIter(s, s+10, 5, TIter::TADereference), TIter());
     test_exceptions(S(), TIter(s, s+10, 6, TIter::TAComparison), TIter());
 	}
+#endif
 }