Extend the deepsleep wait up to 220% of the Watchdog timeout.
One of the current Watchdog timing requirements is to fire BEFORE twice
the timeout value, but at least one target (K64F) is expected to fire
EXACTLY at a doubled timeout value in deepsleep mode. This patch updates
the test to cope with a new deepsleep timing requirement.
Add a watchdog-kicking thread running in the background when the test
suite is handling a failed assertion. A single watchdog kick did not
provide enough time for the greentea communication if the watchdog
reset happened later than expected.
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.