Fix: ctf-testsuite variant-missing-fields

It is valid for an enumeration to contain labels that are missing in the
variant fields. For instance, an enumeration can be used for many
purposes in addition to being used as a selector for a variant, so we
don't want to enforce that a variant needs to have field names for cases
we never expect as enum field input.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>