selftests: net: avoid just another constant wait

[ Upstream commit 691bb4e4 ]

Using hard-coded constant timeout to wait for some expected
event is deemed to fail sooner or later, especially in slow
env.

Our CI has spotted another of such race:
   # TEST: ipv6: cleanup of cached exceptions - nexthop objects          [FAIL]
   #   can't delete veth device in a timely manner, PMTU dst likely leaked

Replace the crude sleep with a loop looking for the expected condition
at low interval for a much longer range.

Fixes: b3cc4f8a

 ("selftests: pmtu: add explicit tests for PMTU exceptions cleanup")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/fd5c745e9bb665b724473af6a9373a8c2a62b247.1706812005.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>