"Non-zero exit status: 3", what is this mean?

`use Test::Nginx::Socket::Lua::Stream;

log_level('warn');
no_long_string();
run_tests();
DATA

=== TEST 1: simple test for ngx_stream_echo_module
--- stream_server_config
content_by_lua_block {
ngx.say("Hello, stream echo!")
}

--- stream_response
Hello, stream echo!

--- no_error_log
[error]`
The above is my test code.The result is:
t/test.t .. nginx main pid[18259] ppid[18258] log_ratelimit[0],log_ratelimit_burst[0],file[src/core/nginx.c],line[547],func[main]
first init pid[18259] ppid[18258] log_ratelimit[0],log_ratelimit_burst[0],p_interval[0,0x7ff019f3f080],p_burst[0,0x7ff019f3f100],file[src/core/nginx.c],line[310],func[ngx_init_log_ratelimit_state_ctl]

ok 1 - TEST 1: simple test for ngx_stream_echo_module - status code ok
ok 2 - TEST 1: simple test for ngx_stream_echo_module - response_body - response is expected (repeated req 0, req 0)
ok 3 - TEST 1: simple test for ngx_stream_echo_module - pattern "[error]" does not match a line in error.log (req 0)
Can't determine section names. Need two sections in first block at t/test.t line 0.
END failed--call queue aborted.
1..3

Looks like your test exited with 3 just after 3.

Dubious, test returned 3 (wstat 768, 0x300)
All 3 subtests passed

Test Summary Report

t/test.t (Wstat: 768 Tests: 3 Failed: 0)
Non-zero exit status: 3
Files=1, Tests=3, 0 wallclock secs ( 0.03 usr 0.01 sys + 0.24 cusr 0.03 csys = 0.31 CPU)
Result: FAIL

Which I can't understand that why the result is failed while all tests are passed.
Where can i find the meaning of "Non-zero exit status: 3"?

Start with t/test.t and the Test::Nginx::Socket::Lua::Stream run_tests entry point, then reproduce the shown output to trace the section-parsing and exit-status handling. Determine why the harness reports exit status 3 after the three tests pass, and document whether the behavior is expected or should be corrected.