tests: pattern "[error]" should not match any line in error.log of TEST 6 t/162-exit-worker.t.

I found a occasinal failures of the test case with TEST_NGINX_USE_HUP enabled, when I specified no_shuffle() and run TEST 6 following TEST 2, I found that this failure is certainly appear.

The reason for this is that when switching between test block (from TEST 2 to TEST 6) using SIGHUP, there are error logs from previous test block that affects the next test block when the old process is not fully exited.

Perhaps we should modify Test::Nginx module to avoid this case.

error log:

#   Failed test 'TEST 6: skip in cache processes (with exit worker and privileged agent) - pattern "[error]" should not match any line in error.log but matches line "2021/03/19 18:55:35 [error] 3947744#0: exit_worker_by_lua_file error: cannot open /home/jiahao/work/org/lua-nginx-module/t/servroot/html/exit_worker.lua: No such file or directory" (req 0)

• nginx version: 1.19.3.1
• lua-nginx-module version: 0.10.19.
• test-nginx 0.29.

Start with t/162-exit-worker.t, especially TEST 2 and TEST 6, and inspect how Test::Nginx switches test blocks with TEST_NGINX_USE_HUP. Reproduce the failure with no_shuffle() and verify that TEST 6 no longer matches error.log lines left by the previous test block.