Check if parallel testing reduces check time in CI

[IndrajeetPatil]

Related to #2295

[MichaelChirico]

Part of the issue is that I think there's a maximum number of jobs that can be running at once -- speeding up individual jobs helps some, but I'd really like to reduce the number of jobs if some jobs are redundant.

[IndrajeetPatil]

Agreed. But what do you think about turning on the parallel testing for {lintr}, in general?
Do you see any downsides to it, apart from the failing workflow, which is likely to be a {testthat} bug?

[MichaelChirico]

if we could have it run in serial by default and parallel according to environment variable on GHA only (not CRAN) I think that's preferable. parallelism is often a trojan horse for headaches if we don't get a huge benefit

[IndrajeetPatil]

Will setting something like this in testthat.R work (parallel on for non-CRAN, but off for CRAN)?

if (Sys.getenv("NOT_CRAN", "true") == "true") {
  Sys.setenv("TESTTHAT_PARALLEL" = "FALSE")
}

[IndrajeetPatil]

I can reproduce this locally as well, so looks like a legit issue.
Will have a deeper look into why this shows up only when tests are run in parallel.

── Error (test-linter_tags.R:106:3): lintr help files are up to date ───────────
Error in `tools::Rd_db("lintr")`: installed help of package 'lintr' is corrupt
Backtrace:
    ▆
 1. └─tools::Rd_db("lintr") at test-linter_tags.R:106:3