Syntactical validation of merged ATXs (#6030)

## Motivation
Closes #5997

Support handling merged ATXs. Only syntactical validation.

Author: poszu

Makefile

@@ -97,7 +97,7 @@ clear-test-cache:
 .PHONY: clear-test-cache
 
 test: get-libs
-	@$(ULIMIT) CGO_LDFLAGS="$(CGO_TEST_LDFLAGS)" gotestsum -- -race -timeout 5m -p 1 $(UNIT_TESTS)
+	@$(ULIMIT) CGO_LDFLAGS="$(CGO_TEST_LDFLAGS)" gotestsum -- -race -timeout 8m -p 1 $(UNIT_TESTS)
 .PHONY: test
 
 generate: get-libs

activation/activation.go

@@ -809,14 +809,14 @@ func (b *Builder) createAtx(
 			NiPosts: []wire.NiPostsV2{
 				{
 					Membership: wire.MerkleProofV2{
-						Nodes:       nipostState.Membership.Nodes,
-						LeafIndices: []uint64{nipostState.Membership.LeafIndex},
+						Nodes: nipostState.Membership.Nodes,
 					},
 					Challenge: types.Hash32(nipostState.NIPost.PostMetadata.Challenge),
 					Posts: []wire.SubPostV2{
 						{
-							Post:     *wire.PostToWireV1(nipostState.Post),
-							NumUnits: nipostState.NumUnits,
+							Post:                *wire.PostToWireV1(nipostState.Post),
+							NumUnits:            nipostState.NumUnits,
+							MembershipLeafIndex: nipostState.Membership.LeafIndex,
 						},
 					},
 				},