Fix genpdu sensor value column and connection detection

genpdu read negative-capable sensors from the unsigned value column,
which the MIB leaves undefined for them: a PX4 answers it with 0 for
inlet reactive power and carries the reading in the signed column. Use
SensorSignedMinimum to pick the column, as the MIB specifies. The
unsigned column is still needed for active energy, whose range exceeds
Integer32 and reads 0 in the signed one.

An unreachable PDU or a wrong credential looked like a PDU without
switched outlets, since the switching probe was the first exchange with
the device. Probe pduCount first (implemented by every PDU2 agent,
verified on PX2, PX3 and PX4) and return no session when it fails, so
callers print "Couldn't connect to <pdu>". Warn when pduCount is not 1
rather than refusing: a linked primary still answers for its own
outlets.

Add unit tests for the genpdu sensor, session and inventory paths.

Signed-off-by: Kilian Cavalotti <kilian@stanford.edu>