mvcc/backend tests: Refactor: Do not mix testing&prod code.

server/mvcc/backend/export_test.go

@@ -0,0 +1,15 @@
+package backend
+
+import bolt "go.etcd.io/bbolt"
+
+func DbFromBackendForTest(b Backend) *bolt.DB {
+	return b.(*backend).db
+}
+
+func DefragLimitForTest() int {
+	return defragLimit
+}
+
+func CommitsForTest(b Backend) int64 {
+	return b.(*backend).Commits()
+}