fix: upstream dns resolver failed response 503

[syzh]

fixed issue #3342.

[spacewander]

Need to update the tests.

[syzh]

Need to update the tests.

I need some time to add more tests case

[spacewander]

@syzh
You just need to update the expected status code in the tests.

[Firstsawyou]

Hi, @syzh

You need to update the test case to solve the problem of ci failure.

For example: TEST 8 of the upstream2.t test case, you need to change 502 to 503.

#   Failed test 'TEST 8: hit empty nodes upstream - status code ok'
#   at /home/runner/work/apisix/apisix/test-nginx/lib/Test/Nginx/Socket.pm line 935.
#          got: '503'
#     expected: '502'
# Looks like you failed 1 test of 30.
t/admin/upstream2.t ....................... 
Dubious, test returned 1 (wstat 256, 0x100)

[membphis]

ping @syzh