exit should return non-zero value to indicate that test failed.

tests/PHPUnit/bootstrap.php

@@ -51,7 +51,7 @@ function checkPiwikSetupForTests()
 4) Edit in phpunit.xml the @REQUEST_URI@ and replace with the webserver path to Piwik, eg. '/dev/piwik/'
 
 Try again and now the tests should run!";
-		exit();
+		exit(1);
 	}
 
 	// Now testing if the webserver is running
@@ -69,4 +69,4 @@ Try again and now the tests should run!";
 			. substr($fetched,0,300) . "...'\n\n";
 		exit;
 	}
-}
+}
\ No newline at end of file