Fix and enable test for datadog_downtime (#8815)
The test has been disabled because it started to fail after an update of
the Datadog API client. The issue itself (id cannot be set in
constructor) and other issues (module name, additional attribute in API
responses) are now fixed. The test is now working fine again.
Fixes#3219.
(cherry picked from commit 593d302f0b)
Co-authored-by: Stefan Birkner <github@stefan-birkner.de>
