ci: move regression_tests.R to testthat subdirectory to avoid execution by `R CMD CHECK`

Refs #69
4 jobs for 69-regression-tests-are-not-functionning in 12 minutes and 29 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #131777
build_patched

00:00:30

 
  Regression
passed #131778
regression_patched

00:00:58

 
  Tests
passed #131780
check_as_cran_patched

00:02:11

passed #131779
check_not_cran_patched

00:08:48