test(material/expansion): fix failing unit test on iOS14 safari

An expansion test seems to be flaky/failing in Safari on iOS14
because the `visibility: hidden` style from the expansion panel
content is not applied immediately. We fix this by enforcing
a style recalulcation like we do in the ripple. An alternative
would have been to switch the test to actual async/await where
we then need to wait at least until the next tick. It seems more
idiomatic to stick with `fakeAsync` here though.

(cherry picked from commit 80d3e28)

Author: devversion

src/material/expansion/expansion.spec.ts

@@ -201,6 +201,11 @@ describe('MatExpansionPanel', () => {
     fixture.detectChanges();
     tick(250);
 
+    // Enforce a style recalculation as otherwise browsers like Safari on iOS 14 require
+    // us to wait until the next tick using actual async/await. Not retrieving the computed
+    // styles would result in the `visibility: hidden` on the expansion content to not apply.
+    getComputedStyle(button).getPropertyValue('visibility');
+
     button.focus();
     expect(document.activeElement).not.toBe(button, 'Expected button to no longer be focusable.');
   }));