selftests: fib_tests: Add option to pause after each test

Add option to pause after each test before cleanup is done. Allows
user to do manual inspection or more ad-hoc testing after each test
with the setup in tact.

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