ManifestParser constructor accesses its first argument, don't pass NULL

The constructor does

  env_ = &state->bindings_;

so env_ is effectively set to offsetof(ManifestParser, bindings_).  This
will blow up if env_ gets dereferenced -- this doesn't seem to happen in
these tests, but it's less confusing with this patch.  Also, passing &state is
consistent with the rest of this test.