t5600: modernize style

t5600: modernize style

This is an old script which could use some updating before
we add to it:

  - use the standard line-breaking:

      test_expect_success 'title' '
              body
      '

  - run all code inside test_expect blocks to catch
    unexpected failures in setup steps

  - use "test_commit -C" instead of manually entering
    sub-repo

  - use test_when_finished for cleanup steps

  - test_path_is_* as appropriate

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>