`ast.py` harness accumulates immortal interned identifiers causing spurious OOMs

[StanFromIreland]

For example this testcase: https://oss-fuzz.com/testcase-detail/6425123073884160

As documented, these "live until interpreter shutdown." It also says that "the number of distinct identifiers and string constants is expected to stay low," but I guess they didn't account for fuzzing. I'm not quite sure how to periodically restart the fuzzer, without causing a hit.

(On the upside, this did technically find a CPython bug!)