tests: shell: Adjust for recent changes in libnftnl
libnftnl commit a96d5a338f24e ("rule: Don't append a newline when
printing a rule") affected nft (and iptables-nft) debug output in that
no extra newline is appended to rule bytecode output anymore. Tolerate
this in the sole test case it breaks by ignoring changes to blank lines.
Signed-off-by: Phil Sutter <[email protected]>