Fix parallel relocation test

[isidorostsa]

In the original test there where two errors:

1. An atomic variable was fetched in a way that introduced race conditions, skipping the throw portion of the code.
2. The testing was more strict than necessary, making guesses about the inner workings of partitioner_with_cleanup. Testing that every created object is destroyed in case of an exception is enough to abide by P1144.

[hkaiser]

LGTM, thanks!

[hkaiser]

bors merge

[bors]

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