Fix LpClassName colllect bug in the RRP abstraction layer

[Ebrix]

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant

Unit tests are done in scapy-red

• I executed the regression tests (using tox)

In the event of a ClassName set on a registry entry, the PR make it so the function does a second roundtrip with the RPC to actually collect the value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.32%. Comparing base (5009233) to head (ab42766).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4971   +/-   ##
=======================================
  Coverage   80.31%   80.32%           
=======================================
  Files         381      381           
  Lines       93630    93630           
=======================================
+ Hits        75202    75206    +4     
+ Misses      18428    18424    -4     

see 8 files with indirect coverage changes

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