[qa] Test that getdata requests work as expected

We should eventually request a transaction from all peers that announce
it (assuming we never receive it).

We should prefer requesting from outbound peers over inbound peers.

Enforce the max tx requests in flight, and the eventual expiry of those
requests.

Test author:    Suhas Daftuar <sdaftuar@gmail.com>
Adjusted by:    MarcoFalke

test/functional/test_runner.py

@@ -91,6 +91,7 @@ BASE_SCRIPTS = [
     'wallet_labels.py',
     'p2p_segwit.py',
     'p2p_timeouts.py',
+    'p2p_tx_download.py',
     'wallet_dump.py',
     'wallet_listtransactions.py',
     # vv Tests less than 60s vv