Add note on unit testing on GitLab to lab4

[janfrancu]

Unit test can be setup also on GitLab, see tutorial from Tamás K. Papp. It boils down to adding a file .gitlab-ci.yml to the root of a repo with the following few lines

image: julia:1.6                # image comes from Docker hub

default:
  script:
    - julia --project=@. -e "import Pkg; Pkg.test()"

Documentation can be hosted in a similar fashion as well.