`testthat-problems.rds` is unconditionally written regardless of `TESTTHAT_PROBLEMS`

[gadenbuie]

When TESTTHAT_PROBLEMS=false, the _problems folder isn't created, but the .rds file is still written

testthat/R/reporter-check.R

Line 71 in 34d9ab9

saveRDS(problems, "testthat-problems.rds", version = 2)

[hadley]

Hmmm, that's an unfortunate naming coincidence, but I think it should be reasonable to also control its writing with the same env var.