Fix signon credentials not being removed on logout - #20567

Description

When using the signon authentication, logging out only destroyed the phpMyAdmin session. The credentials in the single signon session (PMA_single_signon_user, PMA_single_signon_password, ...) were left in place, so readCredentials() read them again on the next request and the user was logged back in without entering any credentials, for example by using the browser's back button.

AuthenticationSignon now overrides logOut(): it opens the single signon session, removes the credentials phpMyAdmin reads from it, switches back to the phpMyAdmin session and then runs the regular logout. Only the PMA_single_signon_* keys phpMyAdmin reads are removed; the rest of the single signon session belongs to the external application and is left untouched.

The new test testAuthLogoutClearsSignonCredentials fails without this change. The two existing logout tests now set SignonSession, which always exists at runtime (it defaults to '' in config.default.php).

Fixes #17457

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