Workaround for delays in serial read in htrun

Caused tests based around serial communication to take longer than
expected. Most notably the echo tests.

Author: geky

TESTS/mbed_drivers/echo/main.cpp

@@ -53,7 +53,7 @@ Case cases[] = {
 };
 
 utest::v1::status_t greentea_test_setup(const size_t number_of_cases) {
-    GREENTEA_SETUP(30, "echo");
+    GREENTEA_SETUP(180, "echo");
     return greentea_test_setup_handler(number_of_cases);
 }