chore(*): refactoring migrations test

Author: kdinev

projects/igniteui-angular/migrations/update-15_1_0/index.spec.ts

@@ -145,9 +145,7 @@ describe(`Update to ${version}`, () => {
             `$custom-scrollbar: scrollbar-theme($size: 10px);`
         );
 
-        const tree = await schematicRunner
-            .runSchematicAsync(migrationName, {}, appTree)
-            .toPromise();
+        const tree = await schematicRunner.runSchematic(migrationName, { shouldInvokeLS: false }, appTree);
 
         expect(tree.readContent('/testSrc/appPrefix/component/test.component.scss')).toEqual(
             `$custom-scrollbar: scrollbar-theme($scrollbar-size: 10px);`
@@ -160,9 +158,7 @@ describe(`Update to ${version}`, () => {
             `$custom-input: input-group-theme($label-floated-background: transparent, $label-floated-disabled-background: transparent);`
         );
 
-        const tree = await schematicRunner
-            .runSchematicAsync(migrationName, {}, appTree)
-            .toPromise();
+        const tree = await schematicRunner.runSchematic(migrationName, { shouldInvokeLS: false }, appTree);
 
         expect(tree.readContent('/testSrc/appPrefix/component/test.component.scss')).toEqual(
             `$custom-input: input-group-theme();`