Increase wait_lsn_timeout in test_wal_receiver.py

[petuhovskiy]

In these tests wait_lsn_timeout was set to 1s which was too low. If something blinks (sk<->ps connection, sk activation, broker) during the test, the test will likely fail because of that. This commit changes the timeout to 3s which should be more stable.

I noticed this error when I made a PR for timeline_manager in safekeepers, this test was flaking because timeline activation was not instant.