community/xapian-core: attempt to enable tests on all arches again

By default, the xapian-core test suite contains several tests which
depend heavily on timing (e.g. qp_scale1) these tests tend to
occasionally fail on some architectures depending on load etc. To
workaround that, disable these timing tests for now.

I only debugged this on s390x but I believe there is a high chance
that this was also the failure mode observed on the other architectures.

See: https://trac.xapian.org/ticket/764#comment:2