Add logic to avoid running PHPUNIT if the project has no phpunit tests

Issue information

From user: jonathan1055

Related to !139 (merged)

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Problem/Motivation

From #3397270: Nightwatch testing against all opted in versions

[the phpunit failure] is only happening because the phpunit job is assumed to always run unless one explicitly opts out by setting SKIP_PHPUNIT='1'. Arguably we should have similar exists logic for PHPUnit, then projects like Decoupled Pages or CKEditor 5 Paste Filter, which have only Nightwatch, would no longer have to set SKIP_PHPUNIT.

In addition to the normal opt-in and skip- rules, Nightwatch tests include

    - exists:
        - tests/src/Nightwatch/**/*.js
        - modules/*/tests/src/Nightwatch/**/*.js
      when: on_success

Proposed resolution

Add similar exists checks in phpunit, to only attempt to run if the project actually has phpunit tests.

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