Fails to build with go 1.18

[andhe]

lmdb-go fails to build when using go 1.18:

src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:341:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:668:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:682:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:696:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:709:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:720:6: dbi declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:769:6: str declared but not used
src/github.com/bmatsuo/lmdb-go/lmdb/example_test.go:770:10: p2 declared but not used

First reported at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1008395