refactor: Split flag and config init into testable steps, and add tests

[dlipovetsky]

What this PR does / why we need it:
Splits the flag and config initialization into testable steps, and adds tests of each.

The previous tests did not help us catch the regression fixed in #494. The new tests do.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

How Has This Been Tested?:

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration and test output

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

[codecov]

Codecov Report

Attention: Patch coverage is 78.87324% with 15 lines in your changes missing coverage. Please review.

Project coverage is 34.40%. Comparing base (f1bbf93) to head (3ed188f).

Files with missing lines	Patch %	Lines
main.go	78.87%	13 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #497      +/-   ##
==========================================
+ Coverage   33.45%   34.40%   +0.95%     
==========================================
  Files          17       17              
  Lines        2236     2267      +31     
==========================================
+ Hits          748      780      +32     
+ Misses       1468     1467       -1     
  Partials       20       20              

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