tests/shell: rework finding tests and add "--list-tests" option

tests/shell: rework finding tests and add "--list-tests" option

Cleanup finding the test files. Also add a "--list-tests" option to see
which tests are found and would run.

Also get rid of the FIND="$(which find)" detection. Which system doesn't
have a working find? Also, we can just fail when we try to use find, and
don't need a check first.

This is still  after "unshare", which will be addressed next.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>