test: `Fix`: adjust "structs not in `Base`" test

* Do not test the exact return type of `Fix{1}(MyStruct, 1)`. That does not seem necessary, given that the intention behind the test is merely ensuring that `Fix` works correctly for a newly-defined `MyStruct`. * Test that `Fix{1}(MyStruct, 1)()` calls `MyStruct(1)`. The motivation is making PkgEval pass for PR JuliaLang/julia#59623.