sstable_test.hh started as collection of utilities shared between the
various `_sstable_test.cc` files. Predictably other tests started using
it as well, among them some that are non boost unit tests. This poses a
problem as if we add the missing boost/test/unit_test.hpp include to
sstable_test.hh these tests will suddenly have missing symbols from
boost::test. To avoid linking boost::test into all these users, extract
utilities more widely used into sstable_utils.hh
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