[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.

tests/Predis/Command/StringSubstrTest.php

@@ -11,8 +11,6 @@
 
 namespace Predis\Command;
 
-use PHPUnit_Framework_TestCase as StandardTestCase;
-
 /**
  * SUBSTR is actually the old name of GETRANGE in version of Redis <= 2.0.
  * This command should be considered obsolete and we will perform any kind