test(datepicker): fix test referencing a changed method name (#12653)

Author: josephperrott

src/lib/datepicker/datepicker.spec.ts

@@ -482,7 +482,7 @@ describe('MatDatepicker', () => {
         testComponent.assignedDatepicker = testComponent.datepicker;
         fixture.detectChanges();
 
-        testComponent.assignedDatepicker.select(toSelect);
+        testComponent.assignedDatepicker._select(toSelect);
         fixture.detectChanges();
         flush();
         fixture.detectChanges();