cubicweb #2538398 [c-c] add-source ldapfeed broken [done]

we get an assertion error

prioritynormal
typebug
done in3.16.1
load0.100
load left0.000
closed by#78a99960286a [ldapfeed] all options of a source should be in the same group, else you get AssertionError on c-c add-source. Closes #2538398