Fix issues with fire rate test related to rounding tie-breaking towards evens

[lL1l1]

Description of the proposed changes

Fixes the fire rate test to account for banker's rounding (tie break towards even numbers), and corrects erroneous fire rate changes from #5845.

Testing done on the proposed changes

Set my ACU's fire rate to 4/0.8 and added a log statement when the projectile is created to make sure it fires every 2/12 ticks.

Checklist

• [x] Changes are annotated, including comments where useful
• [x] Changes are documented in the changelog for the next game version