issue with fig.keep: don't try to remove figures if all should be kept

[cderv]

when using fig.keep with index vector it could happen. This should fix #1579

I took the opportunity of this PR to

1. Refactor a bit with a new internal function rearrange_figs() to apply then logic previously in block_exec()
2. Add some unit tests thanks to this new function
3. Change the style of the other tests

About the unit test, it took me some times to understand the plot logic with recorderplot and low level plot. I believe I got it right, but you set in the start of the test file

op = options(device = function(file = NULL, ...) {
  pdf(file, ...)
  dev.control('enable')  # important! otherwise plots get discarded
})

which is unset with options(op) a bit after

So I am unsure if I can use plot functions below this. It seems to work though.

Last step would have been a refactoring between files as I think it could be interested to have all the plot related function in a file plot.R like the test files.

That is for another time.

[yihui]

Looks good. Thanks!