(maint) reset provider cache between tests

Without this the `provider` mock would leak between tests.

Author: DavidS

spec/puppet/resource_api_spec.rb

@@ -1787,6 +1787,11 @@ def set(_context, changes) end
       allow(provider_class).to receive(:new).and_return(provider)
     end
 
+    after(:each) do
+      # reset cached provider between tests
+      type.instance_variable_set(:@my_provider, nil)
+    end
+
     it { expect { described_class.register_type(definition) }.not_to raise_error }
 
     context 'with the type registered' do