Inherit from DBusTestCase directly in DNS provider test

[knuxify]

This allows the DBus mock server to be properly torn down once the tests are done running. DBusTestCase inherits from unittests' TestCase, so this remains functionally the same.

Without this patch, I was experiencing CI timeouts on Alpine Linux's package builders.

(will also need to be backported to 2.4)

[sonarqubecloud[bot]]

Quality Gate passed

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