tests/test_bluestore_types: cast duration to ceph::timespan

tests/test_bluestore_types: cast duration to ceph::timespan

on FreeBSD + Clang + libc++ + amd64, std::chrono::nanoseconds::rep is `long
long` instead of `long`, so the explictly instantiated template
operator<< operator instances in `src/common/ceph_time.cc` are not able to
cover this use case. so in this change, the duration is casted to
ceph::timespan instead of nanoseconds. so we can ensure that `operator<<` is
always available in this case.

Signed-off-by: Kefu Chai <kchai@redhat.com>