This commit adds a new static method `Timer::RemoveAll()` which
removes all running timers from the `Scheduler`. This method is only
intended for use by by `test_timer` unit test, allowing it to remove
any timers that may be started from constructors of different objects
in an OT instance before starting the unit test. With this change, we
can have timers started from constructors.
To ensure that the `RemoveAll()` is only used by the unit test, it is
defined as a `protected` method which can still be used in the test
which defines and uses `TestTimer` as a sub-class of `Timer`.
Abtin Keshavarzian
f7fd2b0813