Tweak E2E tests supporting GTIN migration

[puntope]

Changes proposed in this Pull Request:

• Fixes E2E tests related to GTIN errors without depending on the release version.
• Additionally adds a test for checking the GTIN field is visible but read-only for users that have the plugin installed <= 2.8.7

Screenshots:

Detailed test instructions:

1. Run the e2e tests locally and see them passing
2. See GH E2E passing
3. Set gla_install_version version as 2.8.7 and go to Product Page
4. GTIN should be read only but visible
5. Set gla_install_version version as 2.9.0 and go to Product Page
6. GTIN should not be visible

Additional details:

Changelog entry

Dev - Tweak E2E tests for GTIN migration in versions > 2.8.7.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.7%. Comparing base (f17e3c0) to head (2af09e4).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##             develop   #2691   +/-   ##
=========================================
  Coverage       64.7%   64.7%           
  Complexity      4667    4667           
=========================================
  Files            799     799           
  Lines          24645   24645           
  Branches        1242    1242           
=========================================
  Hits           15944   15944           
  Misses          8528    8528           
  Partials         173     173           

Flag	Coverage Δ
js-unit-tests	62.4% <ø> (ø)
php-unit-tests	65.3% <100.0%> (ø)

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

Files with missing lines	Coverage Δ
src/HelperTraits/GTINMigrationUtilities.php	75.9% <100.0%> (ø)

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