Compare get_timeout against the last timeout successfully set.
Merge most of the test cases into one. Do not rely on the tests
execution order.
Fix the watchdog_api_stub to keep timeout value set by init function.
-Added the mock class function to mock mbed_assert_internal
-Added the unit test case to test start,kick,stop
-Modified the interface api name from is_alive to process
-added the unit test cases for process