bpo-38659: Properly re-intialize module variables in test_enum (GH-25516)

Previously TestIntEnumConvert and TestStrEnumConvert would end up
converting the module level variables from their regular int form
to a `test.test_enum.X` instance after _convert would run. This
meant that after a single test ran, the next set of _convert
functions would be operating on the enum instances rather than
ints. This would cause some tests such as the one involving format
to fail when running under a mode that repeatedly runs test such
as the refleak finder.