Fix regression tests

Taught bitcoind to close the HTTP connection after it gets a 'stop' command,
to make it easier for the regression tests to cleanly stop.
Move bitcoinrpc files to correct location.
Tidied up the python-based regression tests.
Fix test build after d138598

Building the tests was giving some vague error message about a doubly-defined
symbol.

The solution is to define ShutdownRequested in test_bitcoin.cpp as well
so that init.cpp does not get pulled in.