tests: rename work-tree tests to *work-tree*

"Work tree" or "working tree" is the name of a checked out tree,
"worktree" the name of the command which manages several working trees.
The naming of tests mixes these two, currently:

$ls t/*worktree*
t/t1501-worktree.sh
t/t1509-root-worktree.sh
t/t2025-worktree-add.sh
t/t2026-worktree-prune.sh
t/t2027-worktree-list.sh
t/t2104-update-index-skip-worktree.sh
t/t3320-notes-merge-worktrees.sh
t/t7011-skip-worktree-reading.sh
t/t7012-skip-worktree-writing.sh
t/t7409-submodule-detached-worktree.sh

$grep -l "git worktree" t/*.sh
t/t0002-gitfile.sh
t/t1400-update-ref.sh
t/t2025-worktree-add.sh
t/t2026-worktree-prune.sh
t/t2027-worktree-list.sh
t/t3320-notes-merge-worktrees.sh
t/t7410-submodule-checkout-to.sh

Rename t1501, t1509 and t7409 to make it clear on first glance that they
test work tree related behavior, rather than the worktree command.

t2104, t7011 and t7012 are about the "skip-worktree" flag so that their
name should remain unchanged.

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