Fixes MatIconTestingModule import (#6797)

## Motivation for features / changes
Importing MatIconTestingModule from `@angular/material/icon/testing`
will break 1p sync. Changing this to import from
`webapp/testing/mat_icon_module.ts` instead, in accordance with previous
usage.

Author: hoonji

tensorboard/webapp/widgets/data_table/context_menu_test.ts

@@ -13,8 +13,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 ==============================================================================*/
 
-import {ComponentFixture, TestBed} from '@angular/core/testing';
-import {MatIconTestingModule} from '@angular/material/icon/testing';
+import {TestBed} from '@angular/core/testing';
+import {MatIconTestingModule} from '../../testing/mat_icon_module';
 import {By} from '@angular/platform-browser';
 import {ContextMenuComponent} from './context_menu_component';
 import {ContextMenuModule} from './context_menu_module';