Use sidekiq/testing Worker.clear API in sidekiq_unique_jobs/testing

Instead of reimplementing the logic, we use the upstream `Worker.clear` API
in the `sidekiq_unique_jobs/testing` `Worker.clear` overwrite.

This fixes an issue that causes an infinite loop in `drain_all` after
calling `clear` on a worker class that has it's queue name defined as a
symbol. Because `sidekiq/testing` uses strings as the keys for the
internal state tracking calling `Sidekiq::Queues[queue].clear` cleared a
queue that never contained a job, while leaving jobs in the
"string-indexed" queue.

This only actually works after [this][1] is merged into `sidekiq` until
then the old behaviour remains the same (clearing workers with their
queue name defined as strings is fine).

[1]: sidekiq/sidekiq#5352

lib/sidekiq_unique_jobs/testing.rb

@@ -71,24 +71,25 @@ def use_options(tmp_config = {})
 
         sidekiq_options(old_options)
       end
-
-      #
-      # Clears the jobs for this worker and removes all locks
-      #
-      def clear
-        jobs.each do |job|
-          SidekiqUniqueJobs::Unlockable.unlock(job)
-        end
-
-        Sidekiq::Queues[queue].clear
-        jobs.clear
-      end
     end
 
     #
-    # Prepends deletion of locks to clear_all
+    # Prepends deletion of locks to clear_all and clear
     #
     module Overrides
+      module ClassMethods
+        #
+        # Clears the jobs for this worker and removes all locks
+        #
+        def clear
+          jobs.each do |job|
+            SidekiqUniqueJobs::Unlockable.unlock(job)
+          end
+
+          super
+        end
+      end
+
       #
       # Overrides sidekiq_options on the worker class to prepend validation
       #
@@ -113,5 +114,9 @@ def clear_all
     end
 
     prepend Overrides
+
+    module ClassMethods
+      prepend Overrides::ClassMethods
+    end
   end
 end