autopkgtest: Adding check for accessing syspref folder

Adding a new check which is using strace to detect if the thunderbird
application will try to search for the path

 /usr/lib/thunderbird/defaults/syspref

that we have added by a Debian specific patch. That folder isn't a real
folder later, it's a symlink to

 /etc/thunderbird/pref

were we ship the Debian specific default settings for Thunderbird.

If the check is detecting the path isn't accessed by Thunderbird we need
to adjust the patch

 debian-hacks/Add-another-preferences-directory-for-applications-p.patch