Add test concurrent_drop_truncate_tablespace to isolation2_schedule

Commit 362c48b6 added test
concurrent_drop_truncate_tablespace but it never made it to the
schedule.
Co-authored-by: NAlexandra Wang <lewang@pivotal.io>

src/test/isolation2/isolation2_schedule

@@ -238,3 +238,6 @@ test: oom_startup_memory
 test: restore_memory_accounting_default
 
 test: distributed_transactions
+
+# Test for tablespace
+test: concurrent_drop_truncate_tablespace