Update the watchdog timing requirements:
1. The watchdog should trigger at, or after the timeout value.
2. The watchdog should trigger before twice the timeout value.
Use a SERIAL_FLUSH_TIME_MS value consistent with other tests.
* capitalize driver class name,
* reword test docs for the driver & HAL,
* capitalize Mbed name,
* reword the comments explaining the SERIAL_FLUSH_TIME_MS macro.