autopkgtest: Look for terminfo files under /usr/share/terminfo

Sven Joachimrequested to merge
joachim-guest/unibilium:autopkgtest-fix into debian/sid

The terminfo database in Debian is distributed among the directories /lib/terminfo (ncurses-base) and /usr/share/terminfo (ncurses-term), future releases might relocate all terminfo files to /usr/share/terminfo as per https://bugs.debian.org/1028202.

Closes: #1028231

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