test: fix box/net.box.test.lua result file

This is the regression from a7c855e5
(net.box: fetch '_vcollation' sysview into the module). The result file
was corrupted, because the test sometimes produces incorrect result in
the test case for on_connect / on_disconnect triggers. This issue is
tracked by #4273.

Follow up #3941.

test/box/net.box.result

@@ -2339,14 +2339,14 @@ connected_cnt
 ...
 disconnected_cnt
 ---
-- 2
+- 1
 ...
 conn:close()
 ---
 ...
 disconnected_cnt
 ---
-- 3
+- 2
 ...
 test_run:cmd('stop server connecter')
 ---