Add test case for FollowsFrom clockskew fix

Signed-off-by: Adrian Bogatu <bogatu.adrian@gmail.com>

model/adjuster/clockskew_test.go

@@ -33,6 +33,7 @@ func TestClockSkewAdjuster(t *testing.T) {
 		host                            string
 		adjusted                        int   // start time after adjustment
 		adjustedLogs                    []int // adjusted log timestamps
+		refs                            []model.SpanRef
 	}
 
 	toTime := func(t int) time.Time {
@@ -112,6 +113,13 @@ func TestClockSkewAdjuster(t *testing.T) {
 				{id: 2, parent: 1, startTime: 0, duration: 50, host: "a", adjusted: 0},
 			},
 		},
+		{
+			description: "do not adjust child that follows from parent",
+			trace: []spanProto{
+				{id: 1, parent: 0, startTime: 10, duration: 100, host: "a", adjusted: 10},
+				{id: 2, parent: 1, startTime: 110, duration: 100, host: "a", adjusted: 110, refs: []model.SpanRef{model.SpanRef{RefType: model.FollowsFrom, SpanID: 1}}},
+			},
+		},
 		{
 			description: "do not adjust child that fits inside parent",
 			trace: []spanProto{