test/boost/sstable_datafile_test: sstable_scrub_test: disable key validation
test/boost/sstable_datafile_test: sstable_scrub_test: disable key validation

The test violates clustering key order on purpose to produce a corrupt
sstable (to test scrub). Disable key validation so when we move the
validator into the writer itself in the next patch it doesn't abort the
test.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla