selftests/net: fix waiting time for ipv6_gc test in fib_tests.sh.

ipv6_gc fails occasionally. According to the study, fib6_run_gc() using
jiffies_round() to round the GC interval could increase the waiting time up
to 750ms (3/4 seconds). The timer has a granularity of 512ms at the range
4s to 32s. That means a route with an expiration time E seconds can wait
for more than E * 2 + 1 seconds if the GC interval is also E seconds.

E * 2 + 2 seconds should be enough for waiting for removing routes.

Also remove a check immediately after replacing 5 routes since it is very
likely to remove some of routes before completing the last route with a
slow environment.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240305183949.258473-1-thinker.li@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>