move worktree tests to t24*

move worktree tests to t24*

The 'git worktree' command used to be just another mode in 'git
checkout', namely 'git checkout --to'.  When the tests for the latter
were retrofitted for the former, the test name was adjusted, but the
test number was kept, even though the test is testing a different
command now.  t/README states: "Second digit tells the particular
command we are testing.", so 'git worktree' should have a separate
number just for itself.

Move the worktree tests to t24* to adhere to that guideline. We're
going to make use of the free'd up numbers in a subsequent commit.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>