Prep VTK C++ tests to run in browsers

- enablesr Rendering{Core, OpenGL}CxxTests in browser with vtk.wasm.
- upgrades to CI can be found in the next commit.
- all C++ tests are run with a python script Testing/WebAssembly/runner.py. This script coordinates the entire wasm testing process.
- the runner.py script generates a html file per unit test at runtime, serves the html, js and wasm files with a http server and opens the URL in a browser. It implements some usefule POST/GET actions which enable C++ tests to preload/dump files and forward stdout/stderr/exit codes to the python process.
- this commit introduces vtkEmscriptenTestUtilities which can load data files and baseline images required by a c++ unit test from a browser sandbox.