Column_family_test allows performing private methods on column_family's
sstable_set. It may be useful not only in the boost tests, so it's moved
from test/boost/sstable_test.hh to test/lib/sstable_test_env.hh.
sstable_test.hh includes sstable_test_env.hh, so no includes need to be
changed.
Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>