Add unit tests

[laurakwhit]

What was changed

Added some unit tests for

• duration.ts
• get-workflow-tasks.ts
• label-text-for-history-event.ts

Note: While these tests don't account for every current scenario, they will hopefully lay the groundwork for future testing.

Checklist

1. Close: n/a

2. How was this tested:

   • Add // @ts-ignore above line 19 in get-workflow-tasks.ts
   • Enter the command npm test webview/src/utilities/[...].spec.ts

3. Any docs updates needed? n/a