Add automated test for "Forgot your username"

The manual test is here: Forgot your username

Existing automated coverage

• ForgotUsernameIT#retrieveUsername: fills an email on the forgot username page, clicks Retrieve username, asserts the "If an account is registered with this email, you will receive the account information on" message and the username(s) in the received email

Missing automated coverage

• Start from the Log-in page, click "Forgot your username?" (add a LoginPage page-object method) and assert the ForgotUsernamePage form is displayed
• Assert the confirmation message ends with the email address that was entered

Suggested location: xwiki-platform-core/xwiki-platform-security/xwiki-platform-security-authentication/xwiki-platform-security-authentication-test/xwiki-platform-security-authentication-test-docker/src/test/it/org/xwiki/security/authentication/test/ui/ForgotUsernameIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).