Refactor some code, and increase the unit test code coverage for `src/main.jl`

[DilumAluthge]

Targets #477

[codecov[bot]]

Codecov Report

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

Comparison is base (ef01653) 99.24% compared to head (fac7f58) 99.49%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dpa/merge-queue #481 +/- ## =================================================== + Coverage 99.24% 99.49% +0.25% =================================================== Files 12 12 Lines 395 398 +3 =================================================== + Hits 392 396 +4 + Misses 3 2 -1 ```

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