[tests] Improve the basic framework of our test suite.

We now have a base test case class for Predis (namely PredisTestCase)
grouping various commonly used utility methods shared by all of the
tests in the suite, greatly improving reusability.

NOTE: Backported from v0.9-dev.

tests/Predis/Command/StringStrlenTest.php

@@ -11,13 +11,13 @@
 
 namespace Predis\Command;
 
-use \PHPUnit_Framework_TestCase as StandardTestCase;
+use PredisTestCase;
 
 /**
  * @group commands
  * @group realm-string
  */
-class StringStrlenTest extends CommandTestCase
+class StringStrlenTest extends PredisCommandTestCase
 {
     /**
      * {@inheritdoc}