ethdb/dbtest: addd test suite for ethdb backends (#19960)

- Move the existing tests from memorydb into a generalized testsuite
that can be run by any ethdb backend implementation.
- Add several more test cases to clarify some non-obvious nuances when
implementing a custom ethdb backend, such as the behaviour of
NewIteratorWithPrefix vs NewIteratorWithStart.
- Add leveldb to the testsuite using in-memory storage for fast
execution.

ethdb/leveldb/leveldb_test.go

@@ -0,0 +1,40 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package leveldb
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/ethdb/dbtest"
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/storage"
+)
+
+func TestLevelDB(t *testing.T) {
+	t.Run("DatabaseSuite", func(t *testing.T) {
+		dbtest.TestDatabaseSuite(t, func() ethdb.KeyValueStore {
+			db, err := leveldb.Open(storage.NewMemStorage(), nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			return &Database{
+				db: db,
+			}
+		})
+	})
+}