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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Busca en las pruebas unitarias de las suites WebClient y WebhookClient el helper async_test, especialmente su uso como decorador y las clases base TestCase. Sustituye esos usos por unittest.IsolatedAsyncioTestCase, elimina el uso del método decorador y ejecuta las suites de pruebas afectadas para confirmar que siguen pasando.