ZJIT: Migrate to insta

Migrate from expect-test to insta snapshot testing library. While expect-test
is very small and has a limited surface, it does not handle nextest well; it
while doing inline snapshot updates, it races other test processes to update
the file, leading to frequent "Failed to process macro invocation" errors.

insta handles this by doing batches; it writes to temporary files which can
then be committed in a batch using `cargo insta review` or
`INSTA_UPDATE=always`.

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