Fix failing E2E tests due to login step failure

[annemirasol]

Changes proposed in this Pull Request:

Fixes failing e2e tests by updating the selector for the login button.

The old selector no longer works because a screen reader element, <h1 class="screen-reader-text">Log In</h1>, is being selected instead of the button we want.

Error log:

- Trying to log-in as admin...
User log-in failed, Retrying... 1/1. page.click: Timeout 30000ms exceeded.
Call log:
  - waiting for locator('text=Log In')
  -   locator resolved to 2 elements. Proceeding with the first one: <h1 class="screen-reader-text">Log In</h1>
  - attempting click action
  -   waiting for element to be visible, enabled and stable
  -   element is visible, enabled and stable
  -   scrolling into view if needed
  -   done scrolling
  -   element is outside of the viewport

Testing instructions

E2E tests (Default) do not fail at the login step.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[wjrosa]

That's really nice, Anne! Thank you for working on this 👍