test: fix another net.box failure

This last error
```
[035]  ...
[035]  disconnected_cnt
[035]  ---
[035] -- 1
[035] +- 2
[035]  ...
[035]  conn:close()
[035]  ---
[035]  ...
[035]  disconnected_cnt
[035]  ---
[035] -- 2
[035] +- 3
[035]  ...
[035]  test_run:cmd('stop server connecter')
[035]  ---
[035]
```
Happens because net.box is able to connect to tarantool before it has
finished bootstrap. When connecting, net.box tries to fetch schema
executing a couple of selects, but fails to pass access check since
grants aren't applied yet. This is described in detail in
https://github.com/tarantool/tarantool/issues/2763#issuecomment-499046998
So, alter the test so that it tolerates multiple connection failures.

Closes #4273