Move playground.cpp to a sandbox module

[aliddell]

We can't build tests on unsupported platforms because they require EGrabber.h. We can add the NOTEST flag when we configure in order to get around this, but we could also just set NOTEST=1 when we can't find the EGrabber library.

[nclack]

there's really just one test that needs egrabber.h. It's the "playground". One option is to remove it, since it's not a real test. Another option is to qualify it in the cmake so it only attempts to build if egrabber is found.

In principle, the integration tests shouldn't depend on "egrabber.h", imo.

[aliddell]

I'm generally in favor of putting things like that in a sandbox module that you have to turn on behind a CMake flag. Happy to do this if that's the way you want to take it.