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>