xlate-test: avoid shell entanglements

Feed the nft expected output found in the .txlate test files to
nft -f via pipe/stdin directly without the shell mangling it.

The shell step isn't needed anymore because xtables-translate no longer
escapes quotes.

We only need to remove the "nft '" and trailing "'" because nft doesn't
expect those.

v3: handle multi-line expectations such as libxt_connlimmit.txlate (Phil Sutter)

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>