Extend GainComputer and SmoothedBufferValue Tests

[RachelMaryamLocke]

Gain Computer:

• Test coverage added for both feed-forward and feed-back compressor auto makeup with fixed and variable gain computer parameters

SmoothedBufferValue:

• Test coverage added for using ArenaAllocator with SmoothedBufferValue

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.98%. Comparing base (d7ab49b) to head (98e16a0).

:exclamation: Current head 98e16a0 differs from pull request most recent head ebffa5b. Consider uploading reports for the commit ebffa5b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #504 +/- ## ========================================== - Coverage 90.54% 80.98% -9.56% ========================================== Files 296 207 -89 Lines 9951 6895 -3056 ========================================== - Hits 9010 5584 -3426 - Misses 941 1311 +370 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud