test(core.log): fix unit test case

[leslie-tsang]

What this PR does / why we need it:

Currently APISIX apisix.core.log test case 6 don't work as expect, this PR fixes it.

Both test case config can pass the prove, which was unexpected.

=== TEST 6: print error log with prefix
--- config
    location /t {
        content_by_lua_block {
            local log = require("apisix.core").log.new("test: ")
            log.error("error log")
            log.warn("warn log")
            log.notice("notice log")
            log.info("info log")
            ngx.say("done")
        }
    }
--- log_level: error
--- request
GET /t
--- error_log
error log
--- no_error_log
test: warn log
test: notice log
test: info log

=== TEST 6: print error log with prefix
--- config
    location /t {
        content_by_lua_block {
            local log = require("apisix.core").log
            log.error("error log")
            log.warn("warn log")
            log.notice("notice log")
            log.info("info log")
            ngx.say("done")
        }
    }
--- log_level: error
--- request
GET /t
--- error_log
error log
--- no_error_log
test: warn log
test: notice log
test: info log

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[leslie-tsang]

@spacewander Could you help me to figure out why this PR can't pass the CI ?

Notice: ed test 'TEST 6: print error log with prefix - pattern "[notice]" should not match any line in error.log but matches line "2021/12/01 07:15:32 [notice] 30313#30313: init wasm vm: wasmtime" (req 0)
# 2021/12/01 07:15:32 [error] 30315#30315: *23 [lua] content_by_lua(nginx.conf:202):4: raw error log, client: 127.0.0.1, server: localhost, request: "GET /t HTTP/1.1", host: "localhost"
# '
#   at /home/runner/work/apisix/apisix/test-nginx/lib/Test/Nginx/Socket.pm line 1280.
# Looks like you failed 1 test of 70.

[leslie-tsang]

@membphis ping

[membphis]

@leslie-tsang one pr for one thing ^_^

change this PR to smaller