Fixed test_migration_job_ext_hms timeout (#1678)

## Changes
- Override job cluster in `ext_hms` tests to reuse
`TEST_EXT_HMS_CLUSTER_ID` to avoid timeout. This also allows us to run
this test during regular acceptance tests

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Resolves #1663

### Tests
<!-- How is this tested? Please see the checklist below and also
describe any other relevant tests -->

- [x] manually tested
- [x] added unit tests
- [x] verified on staging environment (screenshot attached)

<img width="1155" alt="image"
src="https://github.com/databrickslabs/ucx/assets/44292934/01a173e8-54e1-46b1-82e1-184375f6ce7a">

Author: nkvuong

tests/integration/test_ext_hms.py

@@ -94,7 +94,7 @@ def sql_backend(ws, env_or_skip) -> SqlBackend:
 
 
 @retried(on=[NotFound, InvalidParameterValue], timeout=timedelta(minutes=5))
-@pytest.mark.parametrize('prepare_tables_for_migration', [('regular')], indirect=True)
+@pytest.mark.parametrize('prepare_tables_for_migration', ['regular'], indirect=True)
 def test_migration_job_ext_hms(ws, installation_ctx, prepare_tables_for_migration, env_or_skip):
     # this test spins up clusters using ext hms cluster policy, which will have a startup time of ~ 7-10m
     # skip this test if not in nightly test job or debug mode
@@ -143,9 +143,16 @@ def test_running_real_assessment_job_ext_hms(
     if os.path.basename(sys.argv[0]) not in {"_jb_pytest_runner.py", "testlauncher.py"}:
         env_or_skip("TEST_NIGHTLY")
 
+    ext_hms_cluster_id = env_or_skip("TEST_EXT_HMS_CLUSTER_ID")
     ext_hms_ctx = installation_ctx.replace(
         skip_dashboards=True,
-        config_transform=lambda wc: dataclasses.replace(wc, override_clusters=None),
+        config_transform=lambda wc: dataclasses.replace(
+            wc,
+            override_clusters={
+                "main": ext_hms_cluster_id,
+                "table_migration": ext_hms_cluster_id,
+            },
+        ),
         extend_prompts={
             r"Instance pool id to be set.*": env_or_skip("TEST_INSTANCE_POOL_ID"),
             r".*Do you want to update the existing installation?.*": 'yes',