regress/subst: demonstrate that SUBST_VARS doesn't pass newlines

Sure, it's an edge case and has not been necessary until now.
Nevertheless it's good to see how this can be done.

Variables having dots in their names are also not handled completely
correct. This also didn't occur in practice since the variable names
passed to SUBST_VARS are usually A-Za-z0-9_ only.

(rillig)