tests: Check received routes count for labeled-unicast with addpath

tests: Check received routes count for labeled-unicast with addpath

Test failed time to time, let's try this way:

```
$ for x in $(seq 1 20); do cp test_bgp_labeled_unicast_addpath.py test_$x.py; done
$ sudo pytest -s -n 20
```

Ran 10 times using this pattern, no failure :shrug:

Before this change, we checked advertised routes, and at some point `=` was
missing from the output, but advertised correctly. Receiving router gets as
much routes as expected to receive.

I reversed checking received routes, not advertised.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>