E2E Tests: Resolve 404s in Cache-Control Headers Directive tests.

[costdev]

There are two 404s that can occur in ​these tests.

You can see that even in ​this successful run, the 404s occurred (the second one is in the next block).

1. ​this block attempts to publish the post. However, this seems to fail. When await logout(); completes and an attempt is made to go to the published URL, a 404 is triggered. It seems that publishPost can be imported, and await publishPost(); can be called between this line and await logout(); to ensure the post is actually published. This is how it's done in ​another test in the suite.

2. ​this line effectively attempts to navigate to http://localhost:8889/wp-admin/wp-admin, producing a 404. This seems to be resolved by using await visitAdminPage( '/' );.

After implementing these changes, ​the E2E run of the PR shows that the Cache-Control headers directives tests simply pass with no 404 output.

Previously, an attempt to view a new post as a logged-out user and an attempt to visit an admin page resulted in 404s.

This change ensures that both URLs are reachable.

[peterwilsoncc]

In 56197:

Build/Test Tools: Prevent 404 errors in the E2E tests.

Modifies the cache control headers tests to prevent 404 errors for each of the tests.

Props costdev.
Fixes #58777.

Commited r56197 / ​https://github.com/WordPress/wordpress-develop/commit/5e14c430a32f51503383b3a24adb5f15a9706061