Playground: testing multi-node networks

[davidjsherman]

This PR proposes end-to-end testing for multi-node networks within a Playground. It uses CMake to build testing scripts that refer to the correct executables. A test for a proposed extension to the playground format is included.

The testing scenario comprises five simulated Thymio-II robots partitioned into three networks.

1. A solo node with a program loaded from an Aesl file
2. A two-node network with separate programs loaded from an external Aesl file
3. A two-node network with separate programs specified from an Aesl network whose definition is inlined in the Playground

The programs are simple beacons, that broadcast an alive message with the id of the node evry three seconds.

Playground test-1-processes.playground illustrates how to build three networks in a five-node playground, using <process> tags to start asebaswitch, asebamassloader, and asebahttp (for inlined Aesl programs).

Playground test-2-networks.playground illustrates a proposed extension to the XML definition, suggested in #624 (comment). Since this test isn't implemented, the CMakeLists.txt declares that this test is expected to fail.

[stephanemagnenat]

One question, are the tests supposed to work on Windows (in msys64)?

[davidjsherman]

Yes, the tests ought to work anywhere.

[stephanemagnenat]

Given the explanation of #624 (comment), what would you like me to do with this PR? Should we put it WIP until we finish the discussion in #624?