tests/qapi-schema: Rename tests from data- to args-

Since every schema entity has 'data', the data- prefix conveys no
information.  These tests actually exercise commands.  Only commands
have arguments, so change the prefix to to args-.

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