Civi\Test - Fix leak in active module-list for headless test

Civi\Test - Fix leak in active module-list for headless test

Steps to reproduce
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Observed on a `dmaster` build while running these steps:

* Run civix's `make-example.sh`
* Or just run some subset involving a headless test, eg
   * civix generate:test --template=headless 'Civi\Civiexample\BarTest'
   * cv api extension.install key=$EXMODULE
   * phpunit8 ./tests/phpunit/Civi/Civiexample/BarTest.php

Before
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Fatal error: Uncaught Error: Class 'Civi\Api4\SearchSegment' not found in
/home/totten/bknix/build/dmaster/web/sites/all/modules/civicrm/ext/search_kit/Civi/Api4/Service/Spec/Provider/SearchSegmentExtraFieldProvider.php
on line 52

After
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Test runs