INFRA-124 - tools/scripts/phpunit - Add error check

The goal is to eventually switch from the embedded phpunit
("civicrm/packages/PHPUnit") to a more standard/standalone copy.  Howeer,
this requires a different CLI (e.g.  "./scripts/phpunit CRM_Foo_BarTest" vs
"phpunit tests/phpunit/CRM/Foo/BarTest.php").

When we make the switch, we'll need some kind of transition for developers
who use the old CLI. This commit provides them with a better error message.

Note: It's tempting to automatically call the standalone phpunit rather than
printing an error; however, there are edge-cases (like unit-tests for
extensions) to consider, and (for now) it's easier to just throw an error
rather than try to think through each of them.