chore: p2p_client.integraion.test fixes (#15634)

Fixes OOM issues in this test that have started appearing after my PR
#15516.
The good news is that the issue has been in the tests, not the
implementation.

What fixed the issues is:

```TS
    jest.restoreAllMocks();
    jest.resetAllMocks();
```

The tests added in #15516 heavily use mocks and spies, which just pushed
the whole thing _over the edge_ because mocks were not cleaned without
the code above.

Calling `restoreAllMocks` is extremely important if we use spies and/or
mocks.
I have monitored the heap size, and on my machine now it doesn't grow
beyond 700MB, and I was able to run the test with half of what CI uses,
i.e., `NODE_OPTIONS="--max-old-space-size=1024" yarn test`

Author: mralj