all: cleanup the APIs for initializing genesis (#25473)

* all: polish tests

* core: apply feedback from Guillaume

* core: fix comment

ethclient/ethclient_test.go

@@ -248,7 +248,7 @@ func generateTestChain() []*types.Block {
 			g.AddTx(testTx2)
 		}
 	}
-	gblock := genesis.ToBlock(db)
+	gblock := genesis.MustCommit(db)
 	engine := ethash.NewFaker()
 	blocks, _ := core.GenerateChain(genesis.Config, gblock, engine, db, 2, generate)
 	blocks = append([]*types.Block{gblock}, blocks...)

ethclient/gethclient/gethclient_test.go

@@ -83,7 +83,7 @@ func generateTestChain() (*core.Genesis, []*types.Block) {
 		g.OffsetTime(5)
 		g.SetExtra([]byte("test"))
 	}
-	gblock := genesis.ToBlock(db)
+	gblock := genesis.MustCommit(db)
 	engine := ethash.NewFaker()
 	blocks, _ := core.GenerateChain(config, gblock, engine, db, 1, generate)
 	blocks = append([]*types.Block{gblock}, blocks...)