Add Work Products to Testing wg page

[luet]

Description

Add links to slides and videos from the talk series for the Testing working group.

Checklist:

• I have previewed changes locally or with CircleCI (runs when PR is created)
• I have completed any content reviews, such as getting input from relevant working groups. If no, please note this and wait to post the PR to the #website channel until the content has been settled.

When you are ready for a technical review/merge, post the for the link for the PR in the US-RSE Slack (#website) to ask for reviewers.

[mrmundt]

Preview: https://output.circle-artifacts.com/output/job/def21702-bcb7-4e4a-ac3e-f6f5a2d4e5ce/artifacts/0/usrse.github.io/wg/testing/index.html