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
Detach test_schema_changes and test_sstable_conforms_to_mutation_source
into individual files. These two take ~10 minutes each, what's left in
origin finishes within 4 minutes alltogether.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
