Issue #25551: Test condition behavior instead of its internals

test_reset_internal_locks was looking at Event's _cond._lock. This
makes it harder to change internals of the Condition object and
makes the test fragile.

The test was added by Nir Soffer in 6108d30dde21.

Patch by Nir Soffer.

diff --cc Lib/test/lock_tests.py
Simple merge