selftests/bpf: Enlarge select() timeout for test_maps

0Day robot observed that it's easily timeout on a heavy load host.
-------------------
 # selftests: bpf: test_maps
 # Fork 1024 tasks to 'test_update_delete'
 # Fork 1024 tasks to 'test_update_delete'
 # Fork 100 tasks to 'test_hashmap'
 # Fork 100 tasks to 'test_hashmap_percpu'
 # Fork 100 tasks to 'test_hashmap_sizes'
 # Fork 100 tasks to 'test_hashmap_walk'
 # Fork 100 tasks to 'test_arraymap'
 # Fork 100 tasks to 'test_arraymap_percpu'
 # Failed sockmap unexpected timeout
 not ok 3 selftests: bpf: test_maps # exit=1
 # selftests: bpf: test_lru_map
 # nr_cpus:8
-------------------
Since this test will be scheduled by 0Day to a random host that could have
only a few cpus(2-8), enlarge the timeout to avoid a false NG report.

In practice, i tried to pin it to only one cpu by 'taskset 0x01 ./test_maps',
and knew 10S is likely enough, but i still perfer to a larger value 30.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210820015556.23276-2-lizhijian@cn.fujitsu.com