Refactor the mining_create_block_template and mining_wait_next_template
helpers in ipc_util.py to return None if they time out or fail. It makes
the test easier to read and provides a more clear error message in case
of a regression.
There were a few spots that didn't use mining_wait_next_template yet,
which now do.
Split the Mining interface test into focused subtests.
Keep the initial tip-change pre-mine check in run_mining_interface_test.
As a result run_block_template_test no longer has newblockref.
Split Mining interface tests into interface_ipc_mining.py and keep
interface_ipc.py for echo + simple inspectors.
Register the new test in test_runner.py.
The setup code around "Create Mining proxy object" is duplicated
in the new test file, but the simple insector checks below it
are not moved.
