Fix gitlab migration unit test
Closes#1837.
The differences in dates can be explained by commit e19b9653ea, which
changed the order in which "created_date" and "updated_date" are
considered.
(cherry picked from commit b0bba20aa4)
Mock HTTP requests in GitLab migration test
This introduces a new utility which can be added to other tests
making HTTP calls to a live service, to cache the responses of this
service in the repository.
(cherry picked from commit 52053b1389)
Enable mocked HTTP responses for GitLab migration test
(cherry picked from commit 19cefc4de2)
Simplify HTTP mocking utility in unit tests
Follow-up to https://codeberg.org/forgejo/forgejo/pulls/1841
(cherry picked from commit ca517c8bb4)
