tests: worktree: retrofit "checkout --to" tests for "worktree add"

tests: worktree: retrofit "checkout --to" tests for "worktree add"

With the introduction of "git worktree add", "git checkout --to" is
slated for removal. Therefore, retrofit linked worktree creation tests
to use "git worktree add" instead.

(The test to check exclusivity of "checkout --to" and "checkout <paths>"
is dropped altogether since it becomes meaningless with retirement of
"checkout --to".)

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>