Actually run assumevalid.py.

assumevalid was merged as part of PR 9484, but was not added to the
test_runner, so is not run even as part of the extended tests.

This commit adds assumevalid to the list of tests in test_runner. It
also clarifies the code in assumevalid considerably.

test/functional/test_runner.py

@@ -110,6 +110,7 @@ EXTENDED_SCRIPTS = [
     'p2p-feefilter.py',
     'rpcbind_test.py',
     # vv Tests less than 30s vv
+    'assumevalid.py',
     'bip65-cltv.py',
     'bip65-cltv-p2p.py',
     'bipdersig-p2p.py',