Fixed run_tu_with_fresh_reporter's to use newer CompositeReporter
[git-p4: depot-paths = "//src/minitest_tu_shim/dev/": change = 8879]
Reactivated the teardown that asserted number of assertions in underlying test object
[git-p4: depot-paths = "//src/minitest_tu_shim/dev/": change = 8880]