Use a fixed SKU number when testing product adapter

[mikkamp]

Changes proposed in this Pull Request:

The change here introduced in WC 9.2, adds a numeric suffix to the SKU when generating products. This is only a default value if the property is not set. So this PR passes a fixed SKU when generating the product so it will be consistent when we compare it in the test results.

Closes #2556

Detailed test instructions:

1. Install latest versions for unit testing bin/install-wp-tests.sh <db_name> <db_user> <db_pass>
2. Run unit tests vendor/bin/phpunit
3. Confirm that the test test_maps_rules_product_fields_sku no longer fails
4. Check the unit tests in this PR against the latest version of WC and ensure it passes

Changelog entry

• Dev - Use a fixed SKU number when testing product adapter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.0%. Comparing base (315fce2) to head (4c9d6db).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##             develop   #2559   +/-   ##
=========================================
  Coverage       65.0%   65.0%           
  Complexity      4588    4588           
=========================================
  Files            475     475           
  Lines          17900   17900           
=========================================
  Hits           11640   11640           
  Misses          6260    6260           

Flag	Coverage Δ
php-unit-tests	65.0% <ø> (ø)

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[martynmjones]

Hey @mikkamp, thanks for the fix.

Confirmed the AttributeMappingWCProductAdapterTest::test_maps_rules_product_fields_sku test passes with the latest version of WC using this branch.