add timeout env variable for tests with slow/heavy load machines (travisCI)

tests/ModbusMaster/MockServerTestCase.php

@@ -23,10 +23,10 @@ abstract class MockServerTestCase extends TestCase
                 $clientData[] = $output;
             });
 
-            if (strpos(PHP_OS, 'WIN') === false) {
+            if (strpos(PHP_OS, 'WIN') === false || getenv('MOCKSERVER_TIMEOUT_USEC') !== false) {
                 // wait to spin up. needed for linux. unnessecary on Windows 10.
                 // Ugly but even with 150ms sleep test run faster on Linux
-                usleep(150000);
+                usleep(getenv('MOCKSERVER_TIMEOUT_USEC') ?: 150000);
             }
             $closure($port);
         });