BF+ENH: a sample "functionality test" + fix for opportunistic cleaning

b06c61f should have fixed introduced by me bug with dependencies being cleaned up, forbidding normal operation.  Now they should be copied upon every checkout (I think this would be better... if ever size/copying becomes an issue -- why not to simply symlink them?)
Let me know if I should stage it as a separate PR so this more monstrous one could be reviewed?

functionality test relies on vbenchtest repository which I linked as a submodule to vbench.  May be ideally it should be RFed into a setup() method which would initiate such a simple repository with few commands... ?
whenever this PR is accepted, vbenchtest repository should probably migrate under pydata/ organization

I also started to move in some pieces which I adopted from original pandas vbench, e.g. collect_benchmarks() and generate_rst_files()