Update async unit tests to use unittest.IsolatedAsyncioTestCase base classes

Currently unit tests employ a helper async_test method defined in in order to run async tests.

As of v3.8, the unittest module supports a base class IsolatedAsyncioTestCase with a similar API to TestCase, but which also accepts coroutines as test functions. We can update unit test implementations to use this instead of TestCase around the time we decide to support the later version of Python as well.

Todo: For test suites where async_test (typically this is used in the decorator pattern i.e. @async_test) is used, replace TestCase with IsolatedAsyncioTestCase and remove decorator method usage.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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Search the unit tests for the WebClient and WebhookClient suites for the async_test helper, especially its decorator usage and TestCase base classes. Replace those uses with unittest.IsolatedAsyncioTestCase, remove the decorator method usage, and run the affected test suites to confirm they still pass.