use subinterpreters better in tests (esp. in regrtest)

[ericsnowcurrently]

[blocked by #21]

Subinterpreters (PEP 554 impl.) would be useful in the test suite:

• ...
• ...

Tasks:

• blocker: implement PEP 554 for use in test suite #21
• helpers (test.support.subinterpreters)
• use in test suite
• adapt regrtest