kselftests/net: adapt the timeout to the largest runtime

timeout in settings is used by each case under the same directory, so
it should adapt to the maximum runtime.

A normally running net/fib_nexthops.sh may be killed by this unsuitable
timeout. Furthermore, since the defect[1] of kselftests framework,
net/fib_nexthops.sh which might take at least (300 * 4) seconds would
block the whole kselftests framework previously.
$ git grep -w 'sleep 300' tools/testing/selftests/net
tools/testing/selftests/net/fib_nexthops.sh:    sleep 300
tools/testing/selftests/net/fib_nexthops.sh:    sleep 300
tools/testing/selftests/net/fib_nexthops.sh:    sleep 300
tools/testing/selftests/net/fib_nexthops.sh:    sleep 300

Enlarge the timeout by plus 300 based on the obvious largest runtime
to avoid the blocking.

[1]: https://www.spinics.net/lists/kernel/msg4185370.html

Signed-off-by: Zhou Jie <[email protected]>
Signed-off-by: Li Zhijian <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: zhijianli88

tools/testing/selftests/net/settings

@@ -1 +1 @@
-timeout=300
+timeout=1500