test/rilmodem: Fix serial number check

Only compare to the emulator's hard-coded serial number when
the '--emulator' argument is given, not when testing in real
hardware.

Fixes https://bugs.launchpad.net/ubuntu/+source/ofono/+bug/1334867 , a regression caused by commit 2a3958695cdc77252e542015954bc192c8f8d04f .