tests: compress the setup tests

New test helpers:

 - setup_repo, to initialize a repository or gitfile pointing to a
   repository, with core.bare and core.worktree set as specified;

 - try_case, to run setup from a given directory and validate the
   result, with GIT_DIR and GIT_WORK_TREE set as specified;

 - try_repo, to initialize a repository and call "try_case" from the
   toplevel and a subdirectory;

 - run_wt_tests, to run a battery of tests that check for sane
   behavior when GIT_WORK_TREE is set to various positions relative to
   the .git dir and cwd.

Use these helpers to make the test shorter, less repetitive, and (one
hopes) easier to understand and modify.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>