tests/py: Avoid duplicate records in *.got files

tests/py: Avoid duplicate records in *.got files

If payloads don't contain family-specific bits, they may sit in a single
*.payload file for all tested families. In such case, nft-test.py will
consequently write dissenting payloads into a single *.got file. To
avoid the duplicate entries, check if a matching record exists already
before writing it out.

Signed-off-by: Phil Sutter <phil@nwl.cc>