Before this patch luatest accepted two ways to register hooks:

* call-style API: `group.before_all(function() ... end)`
* direct assignment: `group.before_all = function() ... end`

The second form does not work correctly with parametrized tests.
We already encountered this on the tarantool side and had to
rewrite such assignments in tarantool/tarantool#8187.

With this change luatest enforces a single, consistent way to
register hooks:

* hooks must be registered via calls like
  `before_each(function() ... end)`,
  `after_each(function() ... end)`,
  `before_all(function() ... end)`,
  `after_all(function() ... end)`,
  `before_suite(function() ... end)`,
  `after_suite(function() ... end)`,
  `before_test('name', function() ... end)` and
  `after_test('name', function() ... end)`;
* assigning to these names (e.g. `group.before_each = fn`) now
  raises an error with a clear message.

Legacy `group.setup` / `group.teardown` remain supported and are
not affected by this change.

Legacy `group.setup` / `group.teardown` are no longer supported;
use `before_each`/`after_each` instead.

Closes tarantool#390