Fixing navDrawer's test-subj when it's animating (#30174)

Previously, when the navDrawer is animating, it's changing it's
data-test-subj immediately, so it appears either collapsed or expanded
before it's reached it's final state. When the navDrawer is animating,
it's not possible to read it's element values using .getVisibleText
(because it's not fully visible yet). This change makes it so that
neither expanded or collapsed is added to the data-test-subj until it's
finished animating.

src/ui/public/chrome/directives/header_global_nav/components/header.tsx

@@ -254,7 +254,11 @@ class HeaderUI extends Component<Props, State> {
             showScrollbar={this.state.showScrollbar}
             data-test-subj={classNames(
               'navDrawer',
-              this.state.isCollapsed ? 'collapsed' : 'expanded'
+              this.state.flyoutIsAnimating
+                ? null
+                : this.state.isCollapsed
+                ? 'collapsed'
+                : 'expanded'
             )}
           >
             <EuiNavDrawerMenu id="navDrawerMenu" onClick={this.onNavClick}>