Version of filetest that runs c++ in multiple processes.

This remake of the c++ template makes each example run in a different process. Globals are not reused and if there is a runtime error, the thing keeps running for the other test cases. Drawbacks:
* Must reread all the sample file for each test case.
* Not sure if it works in Microsoft's yucky c++ compiler (does system() work there?)

I guess the idea is that user would have to set `language.cpp.templateDef.filetest.templateFile = builtin filetest/multiprocess.cpp.tmpl` . I am not sure if this behavior should be default.