Fix test on 32 bit arch

[zhsj]

It can be verified by running

$ GOARCH=386 go test ./...
# github.com/BurntSushi/toml [github.com/BurntSushi/toml.test]
./decode_test.go:281:45: constant 9007199254740992 overflows int
./decode_test.go:282:46: constant -9007199254740992 overflows int
./decode_test.go:292:16: constant 9007199254740991 overflows int
./decode_test.go:293:28: constant -9007199254740991 overflows int
FAIL    github.com/BurntSushi/toml [build failed]
?       github.com/BurntSushi/toml/cmd/toml-test-decoder        [no test files]
?       github.com/BurntSushi/toml/cmd/toml-test-encoder        [no test files]
?       github.com/BurntSushi/toml/cmd/tomlv    [no test files]
?       github.com/BurntSushi/toml/internal     [no test files]
?       github.com/BurntSushi/toml/internal/tag [no test files]
?       github.com/BurntSushi/toml/internal/toml-test   [no test files]
FAIL