Mark TestProvider as deprecated

(up for discussion, depending on adoption of test utils. Alternative approach could be to re-use TestProvider in test-utils)

public/test/helpers/TestProvider.tsx

@@ -20,6 +20,8 @@ export interface Props {
 
 /**
  * Wrapps component in redux store provider, Router and GrafanaContext
+ *
+ * @deprecated Use `@grafana/test-utils` `render` method instead
  */
 export function TestProvider(props: Props) {
   const { store = configureStore(props.storeState), children } = props;