autopkgtests: increase expect timeout to address flaky ldap-user-group-ldap-auth under control

The ldap-user-group-ldap-auth autopkgtest is flaky on Debian CI infrastructure, though so far it couldn't be reproduced on Debian's Salsa nor Ubuntu's test infrastructure. It appears that the test tends to fail under heavy load, or when specific other things are happening on the system. Let's increase the timeout within debian/tests/login.exp, which is used by debian/tests/ldap-user-group-ldap-auth, before marking the test as flaky.

Closes: #1081027