gh-142414: Isolate reloading modules in test_interpreters

[Allactaga]

When module is reloaded it's root level objects are renewed, and if any other module have already imported these variables, they would still be pointing to the old objects. This means that an old object is no longer a dictionary key - new dictionary was constructed. The is checks won't work. And even redefined exception classes won't match in an except clause.

Since the original test was created just to check that we can load module twice, we can do the same inside subinterpreter.

• Issue: ./python -m test test_interpreters test_struct fails with: KeyError: concurrent.interpreters._queues.UNBOUND #142414

[python-cla-bot]

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