Make Date private & make a getter only

Should not be set again = private.
Remove old commented-out tests; match new methods

`getTestIdsInRange` now returns an array instead of a List.
Also cleaned it up; swap dates if 'wrong way around'.
Matched changes in TestInterfaceCli.

Removed old tests in TestInterfaceCli: they're easy to add again, being
so trivial.

Finally, added methods to get IDs matching names (of Tests and
Students)! Should work, needs testing though.
Remove done TODO comment