Begin adding test cases. Fix one bug found so far. More are pending.

FossilOrigin-Name: aadd67ddf2a191629b5356395f75e4556aac904a6e2f6b83742fa4f26e4253a4

src/insert.c

@@ -2338,6 +2338,7 @@ void sqlite3GenerateConstraintChecks(
     ){
       sqlite3VdbeGoto(v, upsertIpkDelay+1);
       sqlite3VdbeJumpHere(v, upsertIpkReturn);
+      upsertIpkReturn = 0;
     }
   }