test(material/schematics): fix ng-add animations test (#24496)

As `addModuleImportToRootModule` is a void function, the test expectations in its result (value undefined) using '.not' would always be true. This change reads the file content and therefore gives the test its sense.

Author: jeripeierSBB

src/material/schematics/ng-add/index.spec.ts

@@ -239,13 +239,15 @@ describe('ng-add schematic', () => {
       // Simulate the case where a developer uses `ng-add` on an Angular CLI project which already
       // explicitly uses the `BrowserAnimationsModule`. It would be wrong to forcibly change
       // to noop animations.
-      const fileContent = addModuleImportToRootModule(
+      addModuleImportToRootModule(
         appTree,
         'BrowserAnimationsModule',
         '@angular/platform-browser/animations',
         project,
       );
 
+      const fileContent = getFileContent(appTree, '/projects/material/src/app/app.module.ts');
+
       expect(fileContent).not.toContain(
         'NoopAnimationsModule',
         'Expected the project app module to not import the "NoopAnimationsModule".',