[EventHubs] Enable DeleteAfter Tag for test resource group

[yunhaoling]

This is a temporary workaround to enable the DeleteAfter tag for the fixture test preparer before we start the work on migrating to eventhub preparer: #12616.

code copied from

azure-sdk-for-python/tools/azure-sdk-tools/devtools_testutils/resource_testcase.py

Lines 61 to 63 in 49befdc

	if self.delete_after_tag_timedelta:
	expiry = datetime.datetime.utcnow() + self.delete_after_tag_timedelta
	parameters['tags'] = {'DeleteAfter': expiry.replace(microsecond=0).isoformat()}

[yunhaoling]

/azp run python - eventhub - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[seankane-msft]

LGTM