DP tolerance needs to be 1IU on each axis

ApproximatelyParallel will only check for parallel within 1 IU, so a 1x1
SEG will always be parallel and the projection is invalid

(cherry picked from commit d0796399a2)

pcbnew/drc/drc_test_provider_diff_pair_coupling.cpp

@@ -202,9 +202,9 @@ static void extractDiffPairCoupledItems( DIFF_PAIR_ITEMS& aDp, DRC_RTREE& aTree
             SEG ssp ( sp->GetStart(), sp->GetEnd() );
             SEG ssn ( sn->GetStart(), sn->GetEnd() );
 
-            // Segments that are == 1 IU in length are approximately parallel with everything and their
-            // parallel projection is < 1 IU, leading to bad distance calculations
-            if( ssp.SquaredLength() > 1 && ssn.SquaredLength() > 1 && ssp.ApproxParallel(ssn) )
+            // Segments that are ~ 1 IU in length per side are approximately parallel (tolerance is 1 IU)
+            // with everything and their parallel projection is < 1 IU, leading to bad distance calculations
+            if( ssp.SquaredLength() > 2 && ssn.SquaredLength() > 2 && ssp.ApproxParallel(ssn) )
             {
                 DIFF_PAIR_COUPLED_SEGMENTS cpair;
                 bool coupled = commonParallelProjection( ssp, ssn, cpair.coupledP, cpair.coupledN );