selftests/bpf: Add test result messages for test_task_storage_map_stress_lookup

selftests/bpf: Add test result messages for test_task_storage_map_stress_lookup

Add test result message when test_task_storage_map_stress_lookup()
succeeds or is skipped. The test case can be skipped due to the choose
of preemption model in kernel config, so export skips in test_maps.c and
increase it when needed.

The following is the output of test_maps when the test case succeeds or
is skipped:

  test_task_storage_map_stress_lookup:PASS
  test_maps: OK, 0 SKIPPED

  test_task_storage_map_stress_lookup SKIP (no CONFIG_PREEMPT)
  test_maps: OK, 1 SKIPPED

Fixes: 73b97bc78b32 ("selftests/bpf: Test concurrent updates on bpf_task_storage_busy")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20220919035714.2195144-1-houtao@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>