hook_test_group_started() and hook_test_group_finished() not used the same way in GUI and scripts/run-tests.sh

In the GUI, hook_test_group_started() and hook_test_group_finished() are called before and after each test class.

(It might be better to call them before and after each test group, as described in #2234479: Deprecate hook_test_* hooks in simpletest).

The behaviour is completely different when running scripts/run-tests.sh:

• hook_test_group_started() is never called
• hook_test_group_finished() is called after each test case, not each test class as in the GUI