Follow-up to 5e0a70683, which got start_node() working. Three more
defects sat behind it:
- initialize_chain() builds the 4-node cache with its own daemon
invocation, which 5e0a70683 did not touch. It therefore still omitted
-regtest (so those cache nodes ran on MAINNET) and -asmap (so they
refused to start at all). Every test that uses the cache -- which is
most of the wallet suite -- died there.
- reindex.py and getblocktemplate_longpoll.py each carried a single
python2 print statement, which is the whole reason they would not even
parse under python3. Shebangs updated to match.
The suite still does not pass: initialize_chain hits a remaining py2
str+int concatenation, and getblocktemplate.py reaches a real test
assertion. Both are beyond this commit, but the harness now gets far
enough to start nodes, answer RPC and begin building the shared chain,
which it could not do before.
Some tests in CheckBlockIndex require chainActive.Tip(), but when reindexing, chainActive has not been set on the first call to CheckBlockIndex.
reindex.py starts a node, mines 3 blocks, stops, and reindexes with CheckBlockIndex enabled.
