Add automated test for "Register an user using email verification"

The manual test is here: Register an user using email verification

Existing automated coverage

• RegisterIT#registerJohnSmith: registration from the register page/modal and login, but with email verification disabled
• XWikiTest#validationKeyStorage: unit only: account validation with a plain or hashed validation key
• DefaultRegistrationConfigurationTest#isEmailValidationRequired: unit only: reading the use_email_verification preference

Missing automated coverage

• Configure mail sending (GreenMail, as in ResetPasswordIT); as admin set Users & Rights > Registration "Use Email Verification" to Yes and Save
• Log out, register U1 with an email address; assert U1 is inactive and cannot log in
• Read the received email, follow the validation link, assert the account is validated and U1 can log in
• Register U2 with the same email; copy the validkey from the email, paste it in the XWiki.AccountValidation form, click "Confirm account" and assert U2 can log in

Suggested location: xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-test/xwiki-platform-administration-test-docker/src/test/it/org/xwiki/administration/test/ui/RegisterIT.java

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