Add missing opt-in-current-rule in d7 template

Problem/Motivation

The opt-in-current-rule is missing in the d7-main.yml template. In D10 this is used to not run 'current' variants of phpunit (and phpstan & nightwatch) when requested not to by setting the variable OPT_IN_TEST_CURRENT: 0. However, when running a D7 pipeline with that variable set to 0 we still get the 'current' variant of the phpunit job.

Proposed resolution

Add the opt-in-current-rule definition exactly like in the D10 template, and use it in .phpunit-base in the same way

Remaining tasks

User interface changes