[3.11] gh-101100: Define `test.regrtest` module to fix references (GH-112381) (#112391)

gh-101100: Define `test.regrtest` module to fix references (GH-112381)

Define test.regrtest module to fix references
(cherry picked from commit d525d01e2794e7e736527eaa7ee309ca1252f5bd)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>