tests/qapi-schema: Improve union discriminator coverage

A union's 'discriminator' must name one of the common members.
QAPISchemaVariants.check() looks it up by its c_name(), then checks
the name matches exactly (because c_name() is not injective).

Tests union-base-empty and union-invalid-discriminator both cover the
case where lookup fails.  Repurpose the latter to cover the case where
it succeeds and the name check fails.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-10-armbru@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo fixed]

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