ci: enable EXPENSIVE for contributor builds

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>