fix vidiffr plot tests on CMD check

[juniperlsimonis]

work locally under testing and on travis under testing, but fail when check is used ref: https://github.com/lionel-/vdiffr/issues/52

[juniperlsimonis]

It looks like it might be a little bit of time before this gets addressed (the issue linked to above describes the situation I'm experiencing), so for the time being, I've set up a way to toggle the testing of the plots for within cran builds to only testing that the plot functions work without throwing errors or warnings. the vidffr tests are still in the testing scripts, but now they're only employed if the tenv variable at the top of the scripts (lda_plot, LDA_TS_plots, TS_plots) to something other than "cran". this allows us to retain the code for testing in the development space but not have it trip up during R CMD check. i've therefore removed the milestone for this for now.

[juniperlsimonis]

this toggle is included in #94