Adjust debian-edu-testsuite to new UID/GID of first user

Guido Berhörsterrequested to merge
personal/gber/ldap-uid-gid into master

This has been changed to UID/GID 2000. Also use getent passwd for obtaining the user account information instead of relying on grep which might match the wrong user.

This must be merged after debian-edu-config!32 (merged).

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