qapi-tests: New tests for union, alternate command arguments

A command's 'data' must be a struct type, given either as a
dictionary, or as struct type name.

Existing test case data-int.json covers simple type 'int'.  Add test
cases for type names referring to union and alternate types.

The latter is caught (good), but the former is not (bug).

Events have the same problem, but since they get checked by the same
code, we don't bother to duplicate the tests.

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