packages,plugins: add package documentation

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/test-utils/api-report.md

@@ -100,6 +100,4 @@ export function wrapInTestApp(
 ): ReactElement;
 
 export * from '@backstage/test-utils-core';
-
-// (No @packageDocumentation comment for this package)
 ```

packages/test-utils/src/index.ts

@@ -13,5 +13,12 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+/**
+ * Utilities to test Backstage plugins and apps.
+ *
+ * @packageDocumentation
+ */
+
 export * from './testUtils';
 export * from '@backstage/test-utils-core';