Rewrite #test_redefinition_mismatch to use a dedicated test class

This test is checking what happens if you try and define a class in a C
extension where that constant is already not a class. It was doing this
by overriding ::Date and then trying to require 'date. The issue with
this is that if we ever add 'date' as a dependency for the test runner,
this test will break because the test runner files get implicitly
required in an `assert_separately` block.

Better use an explicit class for this purpose which can't be accidentally
required elsewhere.

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