[test_gps] Allow to specify custom SUPL server on the command line

For testing purposes it might be useful to specify another
SUPL server than the google default one.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
Merge pull request #240 from philippedeswert/suplserver

[test_gps] Allow to specify custom SUPL server on the command line