Document how to temporarily suppress parallel testing (#2171)

Fixes #2035

Author: hadley

vignettes/parallel.Rmd

@@ -26,6 +26,8 @@ Then add the following line to the `DESCRIPTION`:
 Config/testthat/parallel: true
 ```
 
+If needed (for example, for debugging) you can temporarily suppress parallel testing with `Sys.setenv(TESTTHAT_PARALLEL = "false")`.
+
 By default, testthat will use `getOption("Ncpus", 2)` cores.
 To increase that value for your development machine we recommend setting `TESTTHAT_CPUS` in your `.Renviron`.
 The easiest way to do that is call `usethis::edit_r_environ()` and then add something like the following: