what is a good invalid IP address to use for unit tests?

According to RFC 5737:

The blocks 192.0.2.0/24 (TEST-NET-1), 198.51.100.0/24 (TEST-NET-2),
and 203.0.113.0/24 (TEST-NET-3) are provided for use in documentation.

This means you can use pick an IP address from these ranges:

• 192.0.2.0 – 192.0.2.255
• 198.51.100.0 – 198.51.100.255
• 203.0.113.0 – 203.0.113.255