make the mock provider stoppable

The mock provider couldn't be stopped during diff, because the single
mutex was held through the oepration. Release the mutex so Stop can be
called.

terraform/resource_provider_mock.go

@@ -196,13 +196,14 @@ func (p *MockResourceProvider) Diff(
 	info *InstanceInfo,
 	state *InstanceState,
 	desired *ResourceConfig) (*InstanceDiff, error) {
-	p.Lock()
-	defer p.Unlock()
 
+	p.Lock()
 	p.DiffCalled = true
 	p.DiffInfo = info
 	p.DiffState = state
 	p.DiffDesired = desired
+	p.Unlock()
+
 	if p.DiffFn != nil {
 		return p.DiffFn(info, state, desired)
 	}