tests(*): make nginx_worker_processes to take effect properly

`nginx_worker_processes` is the standard configuration in our template, and it
has an alias `nginx_main_worker_processes`. So, no matter which one we
configure, both will work. However, if we set these options at the same time,
the one that takes effect will be `nginx_main_worker_processes`.

Since our test suite uses
[`nginx_main_worker_processes`](https://github.com/Kong/kong/blob/7a505eaa6870a5
b9e7da914a32464635691f1038/spec/kong_tests.conf#L35), that means using
`nginx_worker_processes` in the test cases is not working.

We have a bunch of test files that are using `nginx_worker_processes` instead of
the `nginx_main_worker_processes` alias, and making one small change to the
kong_test.conf file saves us the trouble of updating all of them (including EE).

spec/kong_tests.conf

@@ -32,7 +32,7 @@ dedicated_config_processing = on
 dns_hostsfile = spec/fixtures/hosts
 resolver_hosts_file = spec/fixtures/hosts
 
-nginx_main_worker_processes = 1
+nginx_worker_processes = 1
 nginx_main_worker_rlimit_nofile = 4096
 nginx_events_worker_connections = 4096
 nginx_events_multi_accept = off