Test sstable::validate() instead. Also rename the unit test testing said
method from scrub_validate_mode_validate_reader_test to
sstable_validate_test to reflect the change.
At this point this test should probably be moved to
sstable_datafile_test.cc, but not in this patch.
Sadly this transition means we loose some test scenarios. Since now we
have to write the invalid data to sstables, we have to drop scenarios
which trigger errors on either the write or read path.
393c42d4a9