Save obtained protocol list before possibly bailing out

When querying number of supported protocols and number of agents supported
by the backend SCMI server under test take care to save such information
before any comparison is made against the expected values for the specific
target at hand.

This will allow us to possibly define a new target platform with relaxed
expectations and just proceed to test any supported protocols it is found.

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