Revert "tests: Fix path to `extcmd_test'"

This reverts commit 4054bcf101.

The changed path to `extcmd_test' is actually invalid because `extcmd_test' is located beneath `<icingaweb>/modules/test/bin'.
I erroneously assumed the latter as invalid because the compiled `extcmd_test' was missing.

test/php/library/Icinga/Protocol/Commandpipe/CommandPipeTest.php

@@ -19,7 +19,7 @@ use Icinga\Module\Monitoring\Command\ScheduleCheckCommand;
 use Icinga\Module\Monitoring\Command\SubmitPassiveCheckresultCommand;
 
 if (!defined('EXTCMD_TEST_BIN')) {
-    define('EXTCMD_TEST_BIN', BaseTestCase::$testDir . '/bin/extcmd_test');
+    define('EXTCMD_TEST_BIN', './bin/extcmd_test');
 }
 
 /**