tests/qapi-schema: Demonstrate misleading optional tag error

tests/qapi-schema: Demonstrate misleading optional tag error

Test flat-union-optional-discriminator declares its union tag as
'*switch': 'Enum', and points to it with 'discriminator': '*switch'.
This gets rejected as "discriminator of flat union 'MyUnion' uses
invalid name '*switch'".  Correct; member 'discriminator' doesn't
accept a '*' prefix.

However, this merely tests name validity checking, which we already
cover elsewhere.  More interesting is testing the valid name 'switch'.
This reports "discriminator 'switch' is not a member of base struct
'Base'", which is misleading.

Copy the existing 'discriminator': '*switch' test to
flat-union-discriminator-bad-name, and rewrite its comment.  Change
flat-union-optional-discriminator to test 'discriminator': 'switch',
and mark it FIXME.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>