test: add interface_ipc_mining.py

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.

test/functional/test_runner.py

@@ -356,6 +356,7 @@ BASE_SCRIPTS = [
     'mempool_cluster.py',
     'feature_logging.py',
     'interface_ipc.py',
+    'interface_ipc_mining.py',
     'feature_anchors.py',
     'mempool_datacarrier.py',
     'feature_coinstatsindex.py',