Add missing Genesis Verification Key for `testing-mainnet`

[jpraynaud]

Content

This PR includes the configuration for the testing-mainnet network Genesis Verification Key which is missing and prevents certificate verification in the Mithril explorer.

Pre-submit checklist

• Branch
  • [x] Tests are provided (if possible)
  • [x] Crates versions are updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

[github-actions[bot]]

Test Results

    3 files  ±0     43 suites  ±0   8m 39s :stopwatch: -7s   985 tests ±0    985 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 079 runs  ±0  1 079 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 5518d1fe. ± Comparison against base commit 9f90571d.