Fix 'DBConnection.OwnershipError' in feature tests.

* Add sandbox config to shared mode when test is async false.

apps/block_scout_web/test/block_scout_web/features/viewing_blocks_test.exs

@@ -1,5 +1,5 @@
 defmodule BlockScoutWeb.ViewingBlocksTest do
-  use BlockScoutWeb.FeatureCase, async: true
+  use BlockScoutWeb.FeatureCase, async: false
 
   alias BlockScoutWeb.{BlockListPage, BlockPage}
   alias Explorer.Chain.Block

apps/block_scout_web/test/block_scout_web/features/viewing_transactions_test.exs

@@ -1,7 +1,7 @@
 defmodule BlockScoutWeb.ViewingTransactionsTest do
   @moduledoc false
 
-  use BlockScoutWeb.FeatureCase, async: true
+  use BlockScoutWeb.FeatureCase, async: false
 
   alias BlockScoutWeb.{AddressPage, TransactionListPage, TransactionLogsPage, TransactionPage}
   alias Explorer.Chain.Wei

apps/block_scout_web/test/support/feature_case.ex

@@ -20,9 +20,13 @@ defmodule BlockScoutWeb.FeatureCase do
     end
   end
 
-  setup do
+  setup tags do
     :ok = Ecto.Adapters.SQL.Sandbox.checkout(Explorer.Repo)
 
+    unless tags[:async] do
+      Ecto.Adapters.SQL.Sandbox.mode(Explorer.Repo, {:shared, self()})
+    end
+
     metadata = Phoenix.Ecto.SQL.Sandbox.metadata_for(Explorer.Repo, self())
     {:ok, session} = Wallaby.start_session(metadata: metadata)
     session = Wallaby.Browser.resize_window(session, 1200, 800)