Fixed #19638 -- Skipped tx support testing for some DBs

PostgreSQL and Oracle always support transactions, so the create table
and rollback test is non-necessary on those DBs.

Thanks to shai for report.

django/db/backends/oracle/base.py

@@ -78,6 +78,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     can_return_id_from_insert = True
     allow_sliced_subqueries = False
     supports_subqueries_in_group_by = False
+    supports_transactions = True
     supports_timezones = False
     supports_bitwise_or = False
     can_defer_constraint_checks = True

django/db/backends/postgresql_psycopg2/base.py

@@ -84,6 +84,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_update_nowait = True
     has_bulk_insert = True
     supports_tablespaces = True
+    supports_transactions = True
     can_distinct_on_fields = True
 
 class DatabaseWrapper(BaseDatabaseWrapper):