Fix a race condition in the validation_chainstatemanager_tests

Summary: The test might run in 2 concurrent processes on some CI builds that check both pre and post activation, causing a race on the file access. Fix this by making the file name unique.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16857