dnsdist: Fix a possible race in the CDB reload regression tests

[rgacogne]

Short description

dnsdist will only automatically reload the CDB file if the mtime changed, but the mtime might not change if the update to the file happens at the exact same second than the first write.

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [ ] compiled this code
• [x] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [x] added or modified regression test(s)
• [ ] added or modified unit test(s)