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>