test: add createNewBlock failure helper

Move the IPC createNewBlock failure assertion into ipc_util.py so
later invalid mining option tests can share the same remote exception check.

test/functional/interface_ipc_mining.py

@@ -30,6 +30,7 @@ from test_framework.wallet import MiniWallet
 from test_framework.p2p import P2PInterface
 from test_framework.ipc_util import (
     assert_capnp_failed,
+    assert_create_new_block_fails,
     destroying,
     load_capnp_modules,
     make_mining_ctx,
@@ -318,11 +319,8 @@ class IPCMiningTest(BitcoinTestFramework):
 
             self.log.debug("Enforce minimum reserved weight for IPC clients too")
             opts.blockReservedWeight = 0
-            try:
-                await mining.createNewBlock(ctx, opts)
-                raise AssertionError("createNewBlock unexpectedly succeeded")
-            except capnp.lib.capnp.KjException as e:
-                assert_capnp_failed(e, "remote exception: std::exception: block_reserved_weight (0) must be at least 2000 weight units")
+            await assert_create_new_block_fails(ctx, mining, opts,
+                "block_reserved_weight (0) must be at least 2000 weight units")
 
         asyncio.run(capnp.run(async_routine()))
 

test/functional/test_framework/ipc_util.py

@@ -162,3 +162,12 @@ async def make_mining_ctx(self):
 def assert_capnp_failed(e, description_prefix):
     assert e.description.startswith(description_prefix), f"Expected description starting with '{description_prefix}', got '{e.description}'"
     assert_equal(e.type, "FAILED")
+
+
+async def assert_create_new_block_fails(ctx, mining, opts, expected_msg):
+    """Assert that mining.createNewBlock fails with the expected remote exception."""
+    try:
+        await mining.createNewBlock(ctx, opts)
+        raise AssertionError("createNewBlock unexpectedly succeeded")
+    except capnp.lib.capnp.KjException as e:
+        assert_capnp_failed(e, f"remote exception: std::exception: {expected_msg}")