test: update cdk table sticky test to work with Safari v15

Updates the CDK table flex-layout sticky to work with Safari v15. The
test starts failing because the `position: sticky` does not work as
expected for the custom elements without an explicit `display`. Updating
the fixture to actually align columns in a row horizontally fixes this
and makes sense, given the name of the fixture being about flex-based table.

Author: devversion

src/cdk/table/table.spec.ts

@@ -2435,6 +2435,9 @@ class StickyPositioningListenerTest implements StickyPositioningListener {
       display: block;
       width: 20px;
     }
+    .cdk-header-row, .cdk-row, .cdk-footer-row {
+      display: flex;
+    }
   `,
   ],
   providers: [{provide: STICKY_POSITIONING_LISTENER, useExisting: StickyFlexLayoutCdkTableApp}],