Fix test mode and write to null

Fix test mode to not always return success.
Don't ask for permission to overwrite `nulmark`.

Before:

    > echo "hello world" > file
    > lz4 -t file
    successfully decoded 12 bytes
    > lz4 -tf file
    successfully decoded 12 bytes
    > lz4 file null
    Warning : /dev/null already exists
    Overwrite ? (Y/n) : n
    Error 12 : No. Operation aborted : /dev/null already exists
    > lz4 file /dev/null
    Warning : /dev/null already exists
    Overwrite ? (Y/n) : n
    Error 12 : No. Operation aborted : /dev/null already exists

After:

    > lz4 -t file
    Error 44 : Unrecognized header : file cannot be decoded
    > lz4 -tf file
    Error 44 : Unrecognized header : file cannot be decoded
    > lz4 file null
    Compressed 12 bytes into 31 bytes ==> 258.33%
    > lz4 file /dev/null
    Compressed 12 bytes into 31 bytes ==> 258.33%

0 files changed, 0 insertions, 0 deletions