tests/run-multitests.py: Add new test runner for multiple Py instances.

This commit adds a test runner and initial test scripts which run multiple
Python/MicroPython instances (eg executables, target boards) in parallel.
This is useful for testing, eg, network and Bluetooth functionality.

Each test file has a set of functions called instanceX(), where X ranges
from 0 up to the maximum number of instances that are needed, N-1.  Then
run-multitests.py will execute this script on N separate instances (eg
micropython executables, or attached boards via pyboard.py) at the same
time, synchronising their start in the right order, possibly passing IP
address (or other address like bluetooth MAC) from the "server" instance to
the "client" instances so they can connect to each other.  It then runs
them to completion, collects the output, and then tests against what
CPython gives (or what's in a provided .py.exp file).

The tests will be run using the standard unix executable for all instances
by default, eg:

    $ ./run-multitests.py multi_net/*.py

Or they can be run with a board and unix executable via:

    $ ./run-multitests.py --instance pyb:/dev/ttyACM0 --instance exec:micropython multi_net/*.py

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