In order to ease maintenance of downstream forks of Testify that would
remove dependency on github.com/stretchr/objx we move all uses of that
module in isolated source files. A fork without that dependency could
just remove those files.
See https://github.com/stretchr/testify/issues/1752#issuecomment-2921545929
The use of objx is quite contained: it is only used in Mock.TestData().
Note that we can't just remove that method or change the return value because
that would be a breaking change.
