Reproduction and potential solution for issue: "cypress-split always uses average duration" #267

[KleaTech]

To reproduce the issue described in #267 :

1. Run npm run timings-alternative-config to generate split file
2. Run npm run timings-alternative-config again to see the durations
3. Observe that all durations are equal (average)
   

To see the fix in action:

1. Run rm test/alternative-config-timings.json to remove the timings file, you need to generate it again
2. Run SPLIT_FIX=true npm run timings-alternative-config to generate split file using the fix
3. Run SPLIT_FIX=true npm run timings-alternative-config again to see the durations
4. Observe that durations are correct