test/rilmodem: Relax revision check

Revision numbers often contain dots (e.g. "1.0"), so relax the check
from "alphanumeric" to "not null".
test/rilmodem: Enable some more checks

Checks for some modem properties (signal strength, revision, serial)
were flagged out earlier, but are now supported by the Ubuntu x86
emulator.
test/rilmodem: Fix Python 3 compatibility

In Python 3, telnetlib reads/writes expect bytes instead of strings.