qapi: Clean up test coverage of simple unions

The tests of UserDefNativeListUnion serve to validate code
generation of simple unions without a base type, except that it
did not have full coverage in the strict test.  The next commits
will remove tests and support for simple unions with a base type,
so there is no real loss at repurposing that test here as
opposed to churn of adding a new test then deleting the old one.

Fix some indentation and long lines while at it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
