Sort ChildReferences in tests

Prior to this change, the `ChildReferences` in tests were sorted
by `PipelineTaskName`. With `Matrix` multiple `ChildReferences`
could have the same `PipelineTaskName` - this causes the tests to
be flakey because the slices are not always sorted the same way.

In this change, we sort the slices of `ChildReferences` by the name
of the `ChildReferences` - mapping to names of `TaskRuns` or `Runs`.
This ensures the slices are always sorted the same way, and removes
the flakes.

Author: jerop

pkg/reconciler/pipelinerun/pipelinerun_updatestatus_test.go

@@ -790,7 +790,10 @@ pipelineTaskName: task
 
 			// Sort the ChildReferences to deal with annoying ordering issues.
 			sort.Slice(actualPrStatus.ChildReferences, func(i, j int) bool {
-				return actualPrStatus.ChildReferences[i].PipelineTaskName < actualPrStatus.ChildReferences[j].PipelineTaskName
+				return actualPrStatus.ChildReferences[i].Name < actualPrStatus.ChildReferences[j].Name
+			})
+			sort.Slice(tc.expectedPrStatus.ChildReferences, func(i, j int) bool {
+				return tc.expectedPrStatus.ChildReferences[i].Name < tc.expectedPrStatus.ChildReferences[j].Name
 			})
 
 			if d := cmp.Diff(tc.expectedPrStatus, actualPrStatus); d != "" {