fix(workflows): exclude abstract test base classes from Mage-OS suite

[rhoerr]

Summary

• PHPUnit 12 treats discovered Abstract*Test.php files as runner warnings ("Class ... is abstract"). The integration job interprets the resulting non-zero PHPUnit exit code as a failure even when every concrete test in the shard passes.
• In the most recent full integration run on mage-os/mageos-magento2 (run 25978664094), ~17 of 29 failed jobs are caused entirely by this — for example TestModuleOverrideConfig, GraphQlCache, Customer, Swatches, Test (Integrity), PaypalGraphQl, CatalogUrlRewrite, LayeredNavigation, several Catalog shards, and the Sales/Bundle shards. Test output looks like OK, but there were issues!.
• This change generates <exclude> entries for any Abstract*Test.php inside the included <directory> shards so only concrete tests load. Single-file shards (*.php matrix entries) are skipped.
• Drive-by: the trailing echo for the integrationIgnore group had unescaped backticks, causing bash to execute integrationIgnore as a command and log integrationIgnore: command not found in every job. Quoted with single quotes.

Test plan

• Re-run Integration Tests - Full Test Suite on a branch of mage-os/mageos-magento2 (e.g. release/3.x) and confirm the previously-failing abstract-only jobs now pass.
• Confirm the Run Integration Tests for Modules step no longer logs integrationIgnore: command not found.
• Verify the generated phpunit.xml.dist (printed by cat $FILE) includes <exclude> entries for the expected Abstract*Test.php files.

🤖 Generated with Claude Code