New execution model

[Tsche]

Instead of a monolithic test application, we should use a runner model instead. This is a lot more flexible and allows us to recompile test TUs with different options (ie with/without coverage).

This is also fundamental to support continuous testing. By detecting file changes in the runner, we can selectively recompile test TUs to see if the changes affected any of our test outcomes.

[Tsche]

Currently test TUs are compiled as shared libraries. Instead, they should be runnable binaries - this greatly simplifies running partial test trees from vscode.