ci: Extract regression tests from standard thestthat procedure

- Regression test are now totally run with `Rscript tests\regression_tests.R`

Refs #69
4 jobs for 69-regression-tests-are-not-functionning in 1 minute and 32 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build_patched #131749

00:00:30

 
  Regression
failed regression_patched #131750

00:01:02

 
  Tests
skipped check_as_cran_patched #131752
skipped check_not_cran_patched #131751
 
Name Stage Failure
failed
regression_patched Regression

── FAILURE (regression.R:16:9): Compare RunModel_GR1A OutputsModel ─────────────
`testVar` not equivalent to `refVar`.
Component “PotEvap”: Mean relative difference: 0.6917171
Component “Precip”: Mean relative difference: 0.4088846
Component “StateEnd”: target is numeric, current is NULL

[ FAIL 1 | WARN 0 | SKIP 0 | PASS 555 ]
ERROR: Job failed: exit status 1