[qa] Add rpc test for segwit

Amended by Pieter Wuille to use multisig 1-of-1 for P2WSH tests, and BIP9
based switchover logic.

Fixes and py3 conversion by Marco Falke.

qa/pull-tester/rpc-tests.py

@@ -136,6 +136,7 @@ testScripts = [
     'invalidtxrequest.py',
     'abandonconflict.py',
     'p2p-versionbits-warning.py',
+    'segwit.py',
     'importprunedfunds.py',
     'signmessages.py',
 ]