test_result: Annotate commits on gitlab

Use gitlab commit status API to annotate commits based on test result
status. If commit is the head of a merge request, gitlab will display
the status on the merge request.

This works by introducing a "Gitlab API Connector", that would perform
API calls using a Personal Token [1].
To have test results annotated, one should select the Gitlab API
Connector on each test suite repository on the test suite.

Also update testXHTML to install erp5_web_service which is now a
dependency of erp5_test_result

1: https://docs.gitlab.com/ce/user/profile/personal_access_tokens.html#creating-a-personal-access-token