selftests: net: fib_rule_tests: fix support for running individual tests

parsing and usage of -t got missed in the previous patch.
this patch fixes it

Fixes: 816cda9a ("selftests: net: fib_rule_tests: add support to select a test to run")
Signed-off-by: Alaa Mohamed <eng.alaamohamedsoliman.am@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>