build warning and test fixes

[yerden]

Some small fixes:

• mask test variables to suppress Go compiler warnings. Currently, we see this:

$ go test
# github.com/bmatsuo/lmdb-go/lmdb_test
./example_test.go:769:6: str declared but not used
./example_test.go:770:10: p2 declared but not used
./example_test.go:720:6: dbi declared but not used
./example_test.go:709:6: dbi declared but not used
./example_test.go:696:6: dbi declared but not used
./example_test.go:682:6: dbi declared but not used
./example_test.go:668:6: dbi declared but not used
./example_test.go:341:6: dbi declared but not used
vet: typecheck failures
FAIL	github.com/bmatsuo/lmdb-go/lmdb [build failed]

• reformat FALLTHRU statement in C code to match compiler expectations. Currently, we see this:

# github.com/bmatsuo/lmdb-go/lmdb
mdb.c: In function ‘mdb_cursor_put’:
mdb.c:6725:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
      if (SIZELEFT(fp) < offset) {
         ^
mdb.c:6731:5: note: here
     case MDB_CURRENT:
     ^~~~

This fixes #126.