Fix tests that may trigger failures for empty test suites

See sebastianbergmann/phpunit#6276

Author: Jean85

tests/Functional/Command/ParallelCommandTest.php

@@ -252,7 +252,10 @@ public function testExecutionWithoutConfiguration(): void
         $commandTester = new CommandTester($command);
         $exitCode = $commandTester->execute([
             'command' => $command->getName(),
-            '--pass-through' => ['--filter=do_not_execute_anything'],
+            '--pass-through' => [
+                '--filter=do_not_execute_anything',
+                '--do-not-fail-on-empty-test-suite',
+            ],
         ]);
 
         $output = $commandTester->getDisplay();

tests/Functional/Runner/RunnerTest.php

@@ -116,7 +116,10 @@ public function testWarning(): void
     public function testNoTestExecutedDoesntGetMistakenAsAbnormalTermination(): void
     {
         $this->setTextFilter('ThreeGreenTestStub.php');
-        $this->loadContainer(['--group=emptyGroup']);
+        $this->loadContainer([
+            '--group=emptyGroup',
+            '--do-not-fail-on-empty-test-suite',
+        ]);
 
         $this->assertEquals(0, $this->executeRunner());