[IS-11] Independent tests for Senders

[N-Nagorny]

test 2.0 -> delete_active_constraints called in set_up_tests
test 2.1 has been removed
test 2.1.1 has been refactored
test 2.2 has been removed
test 2.2.1 has been refactored
test 2.2.3 -> filling {video,audio}_senders in set_up_tests
tests 2.2.3.1, 2.2.3.2, 2.2.4.1, 2.2.4.2, 2.2.5.1, 2.2.5.2, 2.2.6.1, 2.2.6.2, 2.2.7.1, 2.2.7.2 have been refactored
test 2.3 -> filling senders_with_inputs in set_up_tests
test 2.3.1 stays as is
test 2.3.2 has been removed (duplicates tests 1.4 and 1.5)
test 2.3.3 has been removed (duplicates test 2.2.1)
test 2.3.4 has been removed (duplicates test 1.5)
tests 2.3.5.1, 2.3.5.2 were fixed (see code review suggestions in #728)
test 2.4 has been removed (partially duplicates test 2.2.1)

[alabou]

There are so many modifications that the IS-11 implementers must verify the new tests on their implementation first. As these tests seem to no longer follow the IS-11 activity test plan, there is no way to know if the new tests are still compliant with the test plan. Implementers must test first ...