add date/time vignette contents to testthat

@hannesmuehleisen please close if you don't want this for some reason. thanks

add the contents of

http://lubridate.tidyverse.org/ and/or http://r4ds.had.co.nz/dates-and-times.html

to this testthat script

https://github.com/hannesmuehleisen/MonetDBLite/blob/master/tests/testthat/test_03_dplyr.R