selftests: fib_tests: Add a test case for IPv6 garbage collection

Add 1000 IPv6 routes with expiration time (w/ and w/o additional 5000
permanet routes in the background.)  Wait for a few seconds to make sure
they are removed correctly.

The expected output of the test looks like the following example.

> Fib6 garbage collection test
>     TEST: ipv6 route garbage collection [ OK ]

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>