[GTIN] Adding more tests for GTIN migration

[puntope]

Changes proposed in this Pull Request:

Closes part of #2617

• This PR adds tests specifically for the GTIN Migration
• Additionally, this PR adds tests for GTIN Hidden feature
• A bug was found when writing the tests (Core GTIN was overridden in the migration). This PR fixes the bug.

Detailed test instructions:

1. See the tests passing
2. Create a simple product and set a GTIN in Product Inventory tab
3. Set also the GTIN in Google for the WooCommerce tab
4. Start the migration in Connection test page (Start GTIN Migration)
5. See that the GTIN was not overridden

Additional details:

Changelog entry

Dev - Adding tests for GTIN migration tool

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.2%. Comparing base (66dfa38) to head (6910e69).
Report is 9 commits behind head on add/support-core-gtin-field.

Additional details and impacted files

@@                       Coverage Diff                       @@
##             add/support-core-gtin-field   #2679     +/-   ##
===============================================================
+ Coverage                           64.9%   65.2%   +0.3%     
- Complexity                          4670    4671      +1     
===============================================================
  Files                                478     478             
  Lines                              19529   19532      +3     
===============================================================
+ Hits                               12681   12735     +54     
+ Misses                              6848    6797     -51     

Flag	Coverage Δ
php-unit-tests	65.2% <100.0%> (+0.3%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/Jobs/MigrateGTIN.php	88.6% <100.0%> (+88.6%)	⬆️

... and 2 files with indirect coverage changes

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[mikkamp]

Thanks for adding these tests. They look good to me and they all pass as expected.

Based on the report here I noticed that we do not test migrating GTIN's from variable products, nor when an exception is thrown such as a duplicate GTIN.

Since migration is temporary I'm not too worried that this doesn't have full test coverage, but maybe we can at least log an issue with the missing tests.