SiteSpeed: investigate Project File Source test failure

Since 2020-11-19 web_project_file_source started to fail as it was with performance#312 (closed). The issue is to investigate the failure.

Worth noting that the same page didn't break the SiteSpeed pipeline before 2020-11-17:

• Last successful run: https://gitlab.com/gitlab-org/quality/performance-sitespeed/-/pipelines/217644878 - GitLab version: 13.6.0-pre cf5e878aff9
• First failure: https://gitlab.com/gitlab-org/quality/performance-sitespeed/-/pipelines/218259755 - GitLab version: 13.6.0-pre c3f7a9ee0f2