selftests: fib-onlink: Make quiet by default

Add VERBOSE argument to fib-onlink-tests.sh and make output quiet by
default. Add getopt parsing of inputs and support for -v (verbose) and
-p (pause on fail).

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>