fix CRAN note for userspace files in tests

[maxheld83]

Just to keep a record of this here, the issue is:

  * checking for non-standard things in the check directory ... NOTE
 Found the following files/directories:
     �QCards.tex� �mystudy.dat� �mystudy.sta�

CRAN has confirmed:

Please do not write files in the user filespace in examples or tests, rather use tempdir().

The same problem appears to be documented here.

[maxheld83]

I cannot reproduce the NOTE with devtools::check(document = FALSE, args = c('--as-cran')), but it makes sense that some example code produced these files, so I should be able to get rid of them.

[maxheld83]

this should be fixed right on master on my side for the offending detritus from make.cards() and its examples now.

mystudy.* seem to stem from export.pqmethod(), which @aiorazabala wrote.
I couldn't find an easy way to add a tempfile() here, because that'd be a really weird study.name, and I'm not sure what else happens with study.name, that this could break.

So I'll leave that to you @aiorazabala.