There were numerous discussions and issues in the past about the cross-test dependencies that sometimes exist because some tests need to muck with import caches (both on the Python and C level). Some examples:
http://mail.python.org/pipermail/python-dev/2013-January/123409.html
http://mail.python.org/pipermail/python-dev/2013-August/127766.html
Issue #15651 is an example of a hack that has to stay in the code to allow tests to pass. Other things are also difficult - such as simulating situations where some optional modules don't exist (like pyexpat).
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I suggest to add a capability to regrtest that will allow certain tests to request always being run inside a subprocess. Most of the infrastructure is already in place, since this is what regrtest uses to run with -jN today.
An alternative would be to add some test.support utility to help with this, but then test execution would not be uniform between -j1 and -jN and also some things would be difficult to track, such as -R executions (currently all -R runs of a single test have to be within a single process for the refcount tracking to work).