ci: enable EXPENSIVE for contributor builds

Earlier, we enabled EXPENSIVE tests for pushes to integration branches. As we didn't have any CI jobs that run these tests, this was a step in the right direction. It however is an ineffective and inefficient use of the maintainer time, which does not scale, to allow contributors to send changes that are less tested at the list, only to force the maintainer notice breakages caused by their changes but only after these changes are mixed with changes from other contributors. The problematic topic needs to be isolated by bisecting, and it historically has been done by the maintainer alone. It is far better to let the problem identified early, preferably before the problematic code leaves the hands of the original developer. In order for it to happen, the test coverage of the contributor tests must be at least as wide as the coverage of the integration tests. Enable expensive tests for CI jobs triggered by pull requests. This will make each contributor take care of their own, which scales much better. Keep the expensive tests also enabled for the pushes of integration branches, as that is the only place we can notice problems stemming from mismerges and inter-topic interactions, even if the topics from the contributors in isolation all passes these tests. Signed-off-by: Junio C Hamano <gitster@pobox.com>