Fixes bug: The message "[InvalidArgumentException] Test file can't be matched" displayed when exactly one test is running under Windows with the full path to the test.

Oh my god

<?php
   echo '\\'."\n\n";
   $filename = "tests\\unit\WelcomeTest.php";
   echo $filename."\n";
   echo str_replace('\/', '/', $filename)."\n\n";
   $filename = "tests\/unit\/WelcomeTest.php";
   echo $filename."\n";
   echo str_replace('\/', '/', $filename)."\n";

Results:
\

tests\unit\WelcomeTest.php
tests\unit\WelcomeTest.php

tests\/unit\/WelcomeTest.php
tests/unit/WelcomeTest.php