tests: gps: Fix NMEA callback prints

The NMEA sentence string isn't guaranteed to be nul-terminated so we
need to obey the length. While we're at it, let's make sure we only
print NMEA data if it's the correct size.

Signed-off-by: Kalle Vahlman <kalle.vahlman@movial.com>
Merge pull request #72 from kvahlman/nmea-fix

tests: gps: Fix NMEA callback prints