arping(): handle scanning on a unknown IP

[gpotter2]

• handle scanning on an IP that isn't defined in our routing table, if local to the provided interface.

This is a regression, as it was working before we did the overhaul on routing.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (12a3752) to head (249b325).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/l2.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4856   +/-   ##
=======================================
  Coverage   80.88%   80.88%           
=======================================
  Files         368      368           
  Lines       90340    90335    -5     
=======================================
+ Hits        73069    73071    +2     
+ Misses      17271    17264    -7     

Files with missing lines	Coverage Δ
scapy/interfaces.py	96.31% <100.00%> (ø)
scapy/layers/l2.py	84.76% <66.66%> (+0.81%)	⬆️

... and 6 files with indirect coverage changes

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