sstable_directory_test: fix generation in sstable_directory_test_table_scan_incomplete_sstables

The testcase uses an sstable whose mutation key and the generation are
owned by different shards. Due to this, when process_sstable_dir is
called, the sstable gets loaded into a different shard than the one that
was intended. This also means that the sstable and the sstable manager
end up in different shards.

The following patch will introduce a condition variable in sstables
manager which will be signalled from the sstables. If the sstable and
the sstable manager are in different shards, the signalling will cause
the testcase to fail in debug mode with this error : "Promise task was
set on shard x but made ready on shard y". So, fix it by supplying
appropriate generation number owned by the same shard which owns the
mutation key as well.

Signed-off-by: Lakshmi Narayanan Sreethar <lakshmi.sreethar@scylladb.com>
(cherry picked from commit 24064064e9)

test/boost/sstable_directory_test.cc

@@ -195,7 +195,7 @@ SEASTAR_TEST_CASE(sstable_directory_test_table_simple_empty_directory_scan) {
 // Test unrecoverable SSTable: missing a file that is expected in the TOC.
 SEASTAR_TEST_CASE(sstable_directory_test_table_scan_incomplete_sstables) {
     return sstables::test_env::do_with_async([] (test_env& env) {
-        auto sst = make_sstable_for_this_shard(std::bind(new_env_sstable, std::ref(env)));
+        auto sst = make_sstable_for_this_shard(std::bind(new_sstable, std::ref(env), env.tempdir().path().native(), generation_type(this_shard_id())));
 
         // Now there is one sstable to the upload directory, but it is incomplete and one component is missing.
         // We should fail validation and leave the directory untouched