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.

(cherry picked from commit 9809d33)

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".',