test: vlc-demux-libfuzzer: read target from cmdline name

Work-arround oss-fuzz that can't accept scripts like the following:

export VLC_TARGET=$target
exec ./vlc-demux-dec-libfuzzer "\$@"

Therefore, append the target to the binary name.

cf. https://github.com/google/oss-fuzz/issues/1462#issuecomment-393362400