Allow modules to opt in to testing against max PHP version: OPT_IN_TEST_MAX_PHP

Migrated issue

Reported by: wim leers

Related to !75 (merged)

Problem/Motivation

#3396106: Allow modules to opt in to testing against Drupal 11 even before they support it and #3397129: Allow modules to opt in to testing against Drupal previous major, previous minor, next minor targeted Drupal versions, i.e. _TARGET_CORE.

This is about _TARGET_PHP.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Related issue: Issue #3396106

Related issue: Issue #3397129