tests: Add virnetdevtestdata to EXTRA_DIST
    
    In one of my previous commits (49ed6cff9) I've introduced a test
    among with some files stored under virnetdevtestdata folder.
    While this works perfectly within a git tree, the folder was not
    getting into .tar.gz and therefore the dist-check would fail.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>