tests: Test if BGP session is up additionally for route_server_client setup

tests: Test if BGP session is up additionally for route_server_client setup

Lower connect timer to 5 seconds as well.

```
FAILED test_bgp_route_server_client.py::test_bgp_route_server_client - AssertionError: Cannot see BGP GUA next hop from r3 in r1
```

```
2021-12-02 14:41:21,115 INFO: topolog.r1: vtysh command => "show bgp 2001:db8:f::3/128 json"
2021-12-02 14:41:21,115 DEBUG: topolog.r1: LinuxNamespace(r1): cmd_status("['/bin/bash', '-c', 'vtysh  -c "show bgp 2001:db8:f::3/128 json" 2>/dev/null']", kwargs: {'encoding': 'utf-8', 'stdout': -1, 'stderr': -2, 'shell': False, 'stdin': None})
2021-12-02 14:41:21,159 INFO: topolog.r1: vtysh result:
	{
	}
```

At least can't reproduce a failure locally (before managed to catch it).

Ran >2000 times, no failure.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>