t2026: rename worktree prune test

Linked checkouts are known under the name worktree, now. Rename the test
accordingly.

Specifically, this avoids the confusion that t2026 is actually not about
pruning in or with linked checkouts aka worktress but about pruning
worktrees, i.e. about "git worktree prune" rather than "git prune".

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Michael J Gruber