Fix slow unit tests for FilesExt (#1074)

## What changes are proposed in this pull request?
The unit tests for the FilesExt module are very slow because they
exercise the retry pathway using a real clock, resulting in long periods
spent waiting for retries. For unit tests, this is not necessary; they
can use the clock provided in the Config, which is a RealClock by
default in production but can be mocked in tests using FakeClock.

To fix this, I changed the call of `retried` to use the config's clock,
and then I updated the mock Config we provide as a fixture to use the
fake clock.

## How is this tested?
Existing tests should continue to work & pass.
Unit tests should be substantially faster than before.

NO_CHANGELOG=true

Author: mgyucht

databricks/sdk/mixins/files.py

@@ -1337,6 +1337,7 @@ def extended_is_retryable(e: BaseException) -> Optional[str]:
             # where we believe request didn't reach the server
             is_retryable=extended_is_retryable,
             before_retry=before_retry,
+            clock=self._config.clock,
         )(delegate)()
 
     def _open_download_stream(

tests/conftest.py

@@ -8,6 +8,7 @@
 from databricks.sdk.core import Config
 from databricks.sdk.credentials_provider import credentials_strategy
 
+from .clock import FakeClock
 from .integration.conftest import restorable_env  # type: ignore
 
 
@@ -18,7 +19,7 @@ def noop_credentials(_: any):
 
 @pytest.fixture
 def config():
-    return Config(host="http://localhost", credentials_strategy=noop_credentials)
+    return Config(host="http://localhost", credentials_strategy=noop_credentials, clock=FakeClock())
 
 
 @pytest.fixture