gitlab-ci: enable "GIT_TEST_LONG"

gitlab-ci: enable "GIT_TEST_LONG"

Starting with 7a094d68a2 (ci: run expensive tests on push builds to
integration branches, 2026-05-08) we run expensive tests in our CI for
certain events. So far, this has only been wired up for GitHub Workflows
though, which creates a test gap for GitLab CI.

Plug this gap by also making this work for the latter.

Note that these tests cannot be run on the Windows runners, as they only
have 7.5GB of RAM. This is insufficient for some of the EXPENSIVE tests,
so we explicitly disable "GIT_TEST_LONG" on these jobs.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>