Fix test_v005_v2.c extended name suppport checks

VOLTAGE_DOMAIN_ATTRIBUTES returns, amongst other things, if a domain
supports an extended name: in such a case the reply from will contain ONLY
the first 15 chars of the extended name.

When supported, instead, the extended domain name can be queried by issuing
an additional dedicated VOLTAGE_DOMAIN_NAME_GET command.

The current check against ext_name_supp makes no sense from the
specification point of view and it also causes a segfault when running
against TARGET generic_scmi.

Fix by removing the needless check.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>