UnitTest: simplify failure checking for ds-test

[d-xo]

Description

Simplifies the condition that we use for checking failure in symbolic tests. This was initially needed for backwards compatibility reasons, but I think it's been long enough, and any spurious results introduced by this change can be fixed by upgrading ds-test to the latest.

Checklist

• [x] tested locally
• [ ] added automated tests
• [ ] updated the docs
• [x] updated the changelog