Fix race conditions writing snapshots when junit.jupiter.execution.parallel.enabled is true

[nathanregner]

No description provided.

[nedtwigg]

Fantastic contribution, thanks very much!

Can you update the changelog under ## Unreleased https://github.com/diffplug/selfie/blob/main/jvm/CHANGELOG.md. Maybe something like this:

### Fixed
- snapshots created by `junit.jupiter.api.TestFactory` are no longer garbage-collected (#534)
- support parallel testing under `junit.jupiter.execution.parallel.enabled=true` (#534)

I'll merge and release pronto.

[nathanregner]

Fantastic contribution, thanks very much!

Can you update the changelog under ## Unreleased https://github.com/diffplug/selfie/blob/main/jvm/CHANGELOG.md. Maybe something like this:

### Fixed
- snapshots created by `junit.jupiter.api.TestFactory` are no longer garbage-collected (#534)
- support parallel testing under `junit.jupiter.execution.parallel.enabled=true` (#534)

I'll merge and release pronto.

Thank you for the fantastic library! Done!

[nedtwigg]

For future reference, I prefer revisions to get pushed up as new commits rather than force-pushing over old ones. Easier to review just the incremental change rather than the whole thing over. Once it gets pushed up I'd rather the history be "honest" to the conversation from there rather than clean.