ci: update gitlab-ci.yml for regression tests (only for scheduled tests)

- Add intermediate "regression" stage for recording examples outputs of both RCRAN and current version in RDS files
- the regression tests will be automatically added in "tests" stage

Refs #59

.gitlab-ci.yml

 stages:
   - update_packages
   - build
+  - regression
   - tests
 
 default:
@@ -30,6 +31,16 @@ default:
     untracked: true
     expire_in: 1 week
 
+.regression:
+  stage: regression
+  only:
+    refs:
+      - schedules
+  script:
+    - Rscript -e 'source("tests/testthat/store_examples.R"); StoreRefExampleResults("airGR");'
+    - R CMD INSTALL .
+    - Rscript -e 'source("tests/testthat/store_examples.R"); StoreTestExampleResults("airGR");'
+
 .check_not_cran:
   stage: tests
   variables:
@@ -52,6 +63,21 @@ build_patched:
     R_VERSION: "patched"
   extends: .build
 
+regression_patched:
+  variables:
+    R_VERSION: "patched"
+  extends: .regression
+
+regression_devel:
+  variables:
+    R_VERSION: "devel"
+  extends: .regression
+
+regression_oldrel:
+  variables:
+    R_VERSION: "oldrel"
+  extends: .regression
+
 check_not_cran_patched:
   variables:
     R_VERSION: "patched"
@@ -125,3 +151,4 @@ check_as_cran_oldrel:
   variables:
     R_VERSION: "oldrel"
   extends: .check_as_cran
+