MEDIUM: check/spoe: Use SPOP multiplexer to perform SPOP health-checks

MEDIUM: check/spoe: Use SPOP multiplexer to perform SPOP health-checks

The SPOP health-checks are now performed using the SPOP multiplexer. This
will be fixed later, but for now, it is considered as a L4 health-check and
no specific status code is reported. It means the corresponding vtest script
is marked as broken for now.

Functionnaly speaking, the same is performed. A connection is opened, a
HELLO frame is sent to the agent and we wait for the HELLO frame from the
agent in reply. But only L4OK, L4KO or L4TOUT will be reported.

The related issue is #2502.