This commit adds the following features:
- Example posix drivers to support virtual time simulation, allowing tests
to run faster than real time.
- A python-based simulation driver that coordinates the virtual time
simulations. Each alarm event and message transmission across all nodes
are scheduled in a single discrete time event queue. As a result, only
a single node is making forward progress at a time.
- Updated tests to utilize new virtual-time simulator, allowing them to run
faster than real time.
- To enable virtual time feature, define VIRTUAL_TIME=1 env var and set
CPPFLAGS=-DOPENTHREAD_POSIX_VIRTUAL_TIME=1.
Yakun Xu