add tests/buildslave to exercise bfreeze/"frozen"

[warner]

#585 added support for "bbfreeze"-generated single-file executables. I don't know if this still works: there is code (in __init__.py, at least) to detect the presence of sys.frozen, which occurs if you're inside a bbfreeze binary, but I don't think we have anything which tests this.

Maybe a buildslave which creates the frozen executable and then runs the test suite, or at least does a basic smoketest and --version check.