[e2e] Make expandable tree items more actionable

[ozyx]

Summary

We've made great improvements to the a11y of our object tree for use within our e2e tests. For example, we can now use compounding locators such as the following in order to dynamically expand tree items:

const treePane = page.locator('#tree-pane');
const collapsedTreeItems = treePane.locator('role=treeitem[expanded=false]');
// Expand the first collapsed tree item
await collapsedTreeItems.first().locator('.c-disclosure-triangle').click();

However, .c-disclosure-triangle (the actual element we click to expand the tree item) doesn't currently have any visibility css property attached to it, so Playwright locators will pick up expand triangles for unexpandable tree items (such as Clocks) since they are technically invisible to the end user, but not to Playwright's locators.

A fix for this would be to add visibility: hidden or visibility: visible to the .c-disclosure-triangle class depending on whether or not it .is-enabled (another class we already append to expandable tree items).