gh-115490: Work around test.support.interpreters.channels not handling unloading...

gh-115490: Work around test.support.interpreters.channels not handling unloading (#115515)

Work around test.support.interpreters.channels not handling unloading, which
regrtest does when running tests sequentially, by explicitly skipping the
unloading of test.support.interpreters and its submodules.

This can be rolled back once test.support.interpreters.channels supports
unloading, if we are keeping sequential runs in the same process around.