Update Greentea EMAC tests

Increase timeout for EMAC tests. Old timeout (600 seconds) was not
enough for slower devices to complete the tests.

Check that connection was successful when running test case
emac_test_initialize.

TESTS/network/emac/emac_test_initialize.cpp

@@ -102,7 +102,7 @@ void test_emac_initialize()
 #endif
 
     // Power up the interface and emac driver
-    network_interface->connect();
+    TEST_ASSERT_EQUAL_INT(NSAPI_ERROR_OK, network_interface->connect());
 
     worker_loop_link_up_wait();
 }

TESTS/network/emac/main.cpp

@@ -56,7 +56,7 @@ using namespace utest::v1;
 utest::v1::status_t test_setup(const size_t number_of_cases)
 {
 #if !MBED_CONF_APP_ECHO_SERVER
-    GREENTEA_SETUP(600, "default_auto");
+    GREENTEA_SETUP(1200, "default_auto");
 #endif
     return verbose_test_setup_handler(number_of_cases);
 }