Make uninitializedBegin test accurately test its intention

This was previously creating and initializing a tree, and then testing what happened if you created a transaction on a tree ID that definitely didn't exist. What it was trying to test was something different, which is the case where a tree had been created/defined, but was not initialized with an empty log root yet. The test now reflects that. This allows google#3201 to avoid a nil check for something that otherwise will be guaranteed to exist.

integration/storagetest/logtests.go

@@ -146,23 +146,22 @@ func (*logTests) TestSnapshot(ctx context.Context, t *testing.T, s storage.LogSt
 
 func (*logTests) TestReadWriteTransaction(ctx context.Context, t *testing.T, s storage.LogStorage, as storage.AdminStorage) {
 	activeLog := mustCreateTree(ctx, t, as, storageto.LogTree)
-	mustSignAndStoreLogRoot(ctx, t, s, activeLog, &types.LogRootV1{RootHash: []byte{0}})
 
 	tests := []struct {
 		desc          string
-		tree          *trillian.Tree
+		doBefore      func()
 		wantNeedsInit bool
 		wantErr       bool
 		wantLogRoot   []byte
 	}{
 		{
 			desc:          "uninitializedBegin",
-			tree:          logTree(-1),
+			doBefore:      func() {},
 			wantNeedsInit: true,
 		},
 		{
-			desc: "activeLogBegin",
-			tree: activeLog,
+			desc:     "activeLogBegin",
+			doBefore: func() { mustSignAndStoreLogRoot(ctx, t, s, activeLog, &types.LogRootV1{RootHash: []byte{0}}) },
 			wantLogRoot: func() []byte {
 				b, err := (&types.LogRootV1{RootHash: []byte{0}}).MarshalBinary()
 				if err != nil {
@@ -175,7 +174,8 @@ func (*logTests) TestReadWriteTransaction(ctx context.Context, t *testing.T, s s
 
 	for _, test := range tests {
 		t.Run(test.desc, func(t *testing.T) {
-			err := s.ReadWriteTransaction(ctx, test.tree, func(ctx context.Context, tx storage.LogTreeTX) error {
+			test.doBefore()
+			err := s.ReadWriteTransaction(ctx, activeLog, func(ctx context.Context, tx storage.LogTreeTX) error {
 				root, err := tx.LatestSignedLogRoot(ctx)
 				if err != nil && !(err == storage.ErrTreeNeedsInit && test.wantNeedsInit) {
 					t.Fatalf("%v: LatestSignedLogRoot() returned err = %v", test.desc, err)