Save submission url to file (bugfix)

[Hook25]

Description

When submitting to C3, Checkbox receives a submission URL. This url is printed to the console. It is very hard and hackish to retreive this URL from the stdout of the process, therefore this adds a new rule to the exporter that also propagates this URL to a file on disk, printing the location like others do

Resolved issues

N/A

Documentation

N/A

Tests

This adds two new unit tests for the functionality + 1 for a small refactoring

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 48.00%. Comparing base (fc12ed5) to head (3b916a0).
Report is 94 commits behind head on main.

Files with missing lines	Patch %	Lines
checkbox-ng/checkbox_ng/launcher/stages.py	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1561      +/-   ##
==========================================
+ Coverage   47.88%   48.00%   +0.12%     
==========================================
  Files         371      371              
  Lines       39820    39833      +13     
  Branches     6731     6730       -1     
==========================================
+ Hits        19066    19121      +55     
+ Misses      20042    19994      -48     
- Partials      712      718       +6     

Flag	Coverage Δ
checkbox-ng	68.60% <95.83%> (+0.31%)	⬆️

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[boukeas]

Huge thanks for taking care of this. +1