Split target `test` into two targets: `test-bats` and `test-doctest`

This merge request simply splits the test target into two separated targets: test-bats and test-doctest.

Motivation

• It allows the tests suites to be launched in parallel in the GitLab-CI pipeline
• It makes it easier to read the reports, separating internal doctesting (test-doctest) from external tests (test-bats)
• As the project grows, I think it will make it easier for the developers to maintain the tests suites this way

Summary of changes

• The test target in the root Makefile has been replaced by two targets test-bats and test-doctest
• The .gitlab-ci.yml has been updated accordingly, so that there are now two tasks at the test stage: test-doctest and test-bats