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.
Remove explicit calls to sleep.
* capitalize driver class name,
* reword test docs for the driver & HAL,
* capitalize Mbed name,
* reword the comments explaining the SERIAL_FLUSH_TIME_MS macro.
On NUMAKER_PFM_NANO130 target, WDT's clock source is fixed to LIRC, which is much
less accurate than other targets. Enlarge delta define to pass this test.