Tests cannot be filtered by @group when running with _phpunit_concurrent=1

Migrated issue

Reported by: jonathan1055

Related to !47 (merged)

Problem/Motivation

Changes in #3384581: Improve concurrency: replace experimental package with core script mean that when running a phpunit step with _PHPUNIT_CONCURRENT=1 the tests are executed using core's run-tests.sh script instead of a direct call to phpunit. This has created (at least) two separate side-effects

1. Arguments passed via _PHPUNIT_EXTRA are not necessarily common between the two scripts. Switching between _PHPUNIT_CONCURRENT=0 and _PHPUNIT_CONCURRENT=1 is no longer a simple toggle, and the value of _PHPUNIT_EXTRA may need to differ
2. The set of tests to be run can no longer be filtered. This used to be possible by passing --group something

A temporary solution for contib maintainers is to leave _PHPUNIT_CONCURRENT=0 to preserve existing behavior.

Steps to reproduce

Proposed resolution

Add a new template variable _PHPUNIT_TESTGROUPS which can take three types of value:

• --all to execute all tests (this is the pre-filled default)
• a comma-separated list of test @group(s) to execute
• no value (empty) - if the test groups are provided by other customisations, such as running via a parallel job matrix

Remaining tasks

User interface changes

API changes

Data model changes

Related issue: Issue #3384581