Fix 'blocks fingerprinting' test

[luixxiul]

Auditors:

Test Plan: npm run test -- --grep='blocks fingerprinting'

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

related: #8760

[luixxiul]

note: as this test has been usually skipped on Travis due to iframe test errors reported on #8760, in this case please avoid relying on travis results and run the test (npm run test -- --grep='blocks fingerprinting') locally. thanks.

[luixxiul]

CC @srirambv for double check.