Fixes #3450 - Remove is--visible class check, instead block on element being visible

[miketaylr]

No description provided.

[miketaylr]

First run was fine, but I re-ran it and got:

× chrome 84.0.4147.105 on Linux - Reporting with wizard - Learn more modal - closing with Esc (0.233s)
    AssertionError: expected 'popup-modal shadow' to include 'is--visible'
      at Command.<anonymous> @ tests/functional/reporting-issue-wizard-non-auth.js:649:20
      at runMicrotasks @ anonymous
      at Test.Learn more modal - closing with Esc [as test] @ tests/functional/reporting-issue-wizard-non-auth.js:648:12
      @ src/lib/Test.ts:268:51
      at runMicrotasks @ anonymous

[miketaylr]

Ah, OK -- I forgot to apply the same "fix" to the rest of the test.

[miketaylr]

OK, green on 2 re-spins and 2 runs locally. r=me