Add GPIO NC test

Check two items of defined behaviour - that you can initialise a
gpio_t with NC, and you can detect that state with gpio_is_connected().

Author: kjbracey

TESTS/mbed_hal/gpio/main.cpp

@@ -0,0 +1,57 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2019 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "utest/utest.h"
+#include "unity/unity.h"
+#include "greentea-client/test_env.h"
+
+using namespace utest::v1;
+
+#include "PinNames.h"
+#include "gpio_api.h"
+
+static void gpio_nc_test()
+{
+    gpio_t nc_obj;
+    gpio_init(&nc_obj, NC);
+    TEST_ASSERT_FALSE(gpio_is_connected(&nc_obj));
+
+    gpio_t led_obj;
+    gpio_init(&led_obj, LED1);
+    if (LED1 == NC) {
+        TEST_ASSERT_FALSE(gpio_is_connected(&led_obj));
+    } else {
+        TEST_ASSERT_TRUE(gpio_is_connected(&led_obj));
+    }
+}
+
+Case cases[] = {
+    Case("gpio NC test", gpio_nc_test)
+};
+
+utest::v1::status_t greentea_test_setup(const size_t number_of_cases)
+{
+    GREENTEA_SETUP(20, "default_auto");
+    return greentea_test_setup_handler(number_of_cases);
+}
+
+Specification specification(greentea_test_setup, cases, greentea_test_teardown_handler);
+
+int main()
+{
+    Harness::run(specification);
+}

hal/gpio_api.h

@@ -31,8 +31,8 @@ extern "C" {
  * \defgroup hal_gpio GPIO HAL functions
  *
  * # Defined behavior
- * * ::gpio_init and other init functions can be called with NC or a valid PinName for the target.
- * * ::gpio_is_connected can be used to test whether a gpio_t object was initialized with NC.
+ * * ::gpio_init and other init functions can be called with NC or a valid PinName for the target - Verified by ::gpio_nc_test
+ * * ::gpio_is_connected can be used to test whether a gpio_t object was initialized with NC - Verified by ::gpio_nc_test
  *
  * # Undefined behavior
  * * Calling any ::gpio_mode, ::gpio_dir, ::gpio_write or ::gpio_read on a gpio_t object that was initialized