feat(stf/branch): simplify merged iterator (#22131)

Co-authored-by: marbar3778 <marbar3778@yahoo.com>

tests/go.mod

@@ -32,7 +32,7 @@ require (
 )
 
 require (
-	cosmossdk.io/core/testing v0.0.0-20240923163230-04da382a9f29
+	cosmossdk.io/core/testing v0.0.0
 	cosmossdk.io/runtime/v2 v2.0.0-20240911143651-72620a577660
 	cosmossdk.io/server/v2/stf v0.0.0-00010101000000-000000000000
 	cosmossdk.io/store/v2 v2.0.0-00010101000000-000000000000
@@ -248,6 +248,7 @@ replace (
 	cosmossdk.io/api => ../api
 	cosmossdk.io/client/v2 => ../client/v2
 	cosmossdk.io/collections => ../collections
+	cosmossdk.io/core/testing => ../core/testing
 	cosmossdk.io/indexer/postgres => ../indexer/postgres
 	cosmossdk.io/runtime/v2 => ../runtime/v2
 	cosmossdk.io/server/v2/appmanager => ../server/v2/appmanager

tests/go.sum

@@ -194,8 +194,6 @@ cloud.google.com/go/workflows v1.6.0/go.mod h1:6t9F5h/unJz41YqfBmqSASJSXccBLtD1V
 cloud.google.com/go/workflows v1.7.0/go.mod h1:JhSrZuVZWuiDfKEFxU0/F1PQjmpnpcoISEXH2bcHC3M=
 cosmossdk.io/core v1.0.0-alpha.5 h1:McjYXAQ6XcT20v2uHyH7PhoWH8V+mebzfVFqT3GinsI=
 cosmossdk.io/core v1.0.0-alpha.5/go.mod h1:3u9cWq1FAVtiiCrDPpo4LhR+9V6k/ycSG4/Y/tREWCY=
-cosmossdk.io/core/testing v0.0.0-20240923163230-04da382a9f29 h1:NxxUo0GMJUbIuVg0R70e3cbn9eFTEuMr7ev1AFvypdY=
-cosmossdk.io/core/testing v0.0.0-20240923163230-04da382a9f29/go.mod h1:8s2tPeJtSiQuoyPmr2Ag7meikonISO4Fv4MoO8+ORrs=
 cosmossdk.io/depinject v1.1.0 h1:wLan7LG35VM7Yo6ov0jId3RHWCGRhe8E8bsuARorl5E=
 cosmossdk.io/depinject v1.1.0/go.mod h1:kkI5H9jCGHeKeYWXTqYdruogYrEeWvBQCw1Pj4/eCFI=
 cosmossdk.io/errors v1.0.1 h1:bzu+Kcr0kS/1DuPBtUFdWjzLqyUuCiyHjyJB6srBV/0=