test: move make_mining_ctx to ipc_util.py

The async routines in both interface_ipc.py and interface_ipc_mining.py
contain redundant code to initialize the mining proxy object.

Move the make_mining_ctx helper into test_framework/ipc_util.py and
update both test files to use it. This removes the boilerplate and
prevents code duplication across the IPC test suite.

test/functional/test_framework/ipc_util.py

@@ -148,3 +148,10 @@ async def mining_get_coinbase_tx(block_template, ctx) -> CoinbaseTxData:
         requiredOutputs=[bytes(output) for output in template_capnp.requiredOutputs],
         lockTime=int(template_capnp.lockTime),
     )
+
+async def make_mining_ctx(self):
+    """Create IPC context and Mining proxy object."""
+    ctx, init = await make_capnp_init_ctx(self)
+    self.log.debug("Create Mining proxy object")
+    mining = init.makeMining(ctx).result
+    return ctx, mining