Issue #3182653 by longwave, Eli-T, devad: PHPUnit 9.4.3 Fatal error: Trait...

Issue #3182653 by longwave, Eli-T, devad: PHPUnit 9.4.3 Fatal error: Trait 'Prophecy\PhpUnit\ProphecyTrait' not found

(cherry picked from commit cbccba94)

core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit9/TestCompatibilityTrait.php

@@ -4,6 +4,12 @@
 
 use Prophecy\PhpUnit\ProphecyTrait;
 
+// @todo Replace with a proper dependency when we stop supporting PHPUnit 8.
+if (!trait_exists(ProphecyTrait::class)) {
+  print "Drupal requires Prophecy PhpUnit when using PHPUnit 9 or greater. Please use 'composer require --dev phpspec/prophecy-phpunit:^2' to ensure that it is present.\n";
+  exit(1);
+}
+
 /**
  * Drupal's forward compatibility layer with multiple versions of PHPUnit.
  */