test: fix backward compatibility breaking for php < 5.5

PHP Parse error:  syntax error, unexpected 'class' (T_CLASS)

http://php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
```
::class
Since PHP 5.5, the class keyword is also used for class name resolution.
```

tests/Bootstrap.php

@@ -1,8 +1,9 @@
 <?php
 $loader = include __DIR__ . '/../vendor/autoload.php';
 
-use PHPUnit\Framework\TestCase;
+$psr4TestCaseClass = 'PHPUnit\Framework\TestCase';
+$psr0TestCaseClass = 'PHPUnit_Framework_TestCase';
 
-if (!class_exists(PHPUnit_Framework_TestCase::class) && class_exists(TestCase::class)) {
-    class_alias(TestCase::class, PHPUnit_Framework_TestCase::class);
+if (!class_exists($psr0TestCaseClass) && class_exists($psr4TestCaseClass)) {
+    class_alias($psr4TestCaseClass, $psr0TestCaseClass);
 }