test: add feature_asmap functional tests

to verify node behaviour and debug log when launching bitcoind in these cases:

1. `bitcoind` with no -asmap arg, using /16 prefix for IP bucketing

2. `bitcoind -asmap=<relative path>`, using the unit test skeleton asmap

3. `bitcoind -asmap/-asmap=` with no file specified, using the default asmap

4. `bitcoind -asmap` with no file specified, and a missing default asmap file

The tests are order-independent. The slowest test (missing default asmap file)
is placed last.

test/functional/test_runner.py

@@ -101,6 +101,7 @@ BASE_SCRIPTS = [
     'rpc_txoutproof.py',
     'wallet_listreceivedby.py',
     'wallet_abandonconflict.py',
+    'feature_asmap.py',
     'feature_csv_activation.py',
     'rpc_rawtransaction.py',
     'wallet_address_types.py',