PopBeforeSmtpTest: skip on Windows

The shell commands used in the test are not available on Windows, so these tests would always fail, so we may as well skip them.

test/POP3/PopBeforeSmtpTest.php

@@ -47,6 +47,16 @@ final class PopBeforeSmtpTest extends TestCase
         }
     }
 
+    /**
+     * Run before each test is started.
+     */
+    protected function set_up()
+    {
+        if (DIRECTORY_SEPARATOR === '\\') {
+            $this->markTestSkipped('This test needs a non-Windows OS to run');
+        }
+    }
+
     /**
      * Run after each test is completed.
      */