e2e: Fix broken tests in trunk against WP versions under 6.9

[priethor]

What

Fixes e2e tests that are broken in trunk after #259

Why

The refactor introduced in #259 was incompatible with WP 6.8, but I didn't catch the test regression as the PR was running the tests against trunk, not 6.8.

How

Only trying to open the metabox panel if the button to open it exists, which will support both cases where the metaboxes panel is open and collapsed.

[github-actions]

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Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.37%. Comparing base (d7a87b1) to head (d141230).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##             trunk    #267   +/-   ##
=======================================
  Coverage     9.37%   9.37%           
  Complexity    4288    4288           
=======================================
  Files          266     266           
  Lines        33159   33159           
=======================================
  Hits          3108    3108           
  Misses       30051   30051           

Flag	Coverage Δ
e2e	21.83% <ø> (ø)
phpunit	6.08% <ø> (ø)

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[priethor]

Confirming the fix works with WP latest (6.8 as of now), which was failing in trunk. Reverting the e2e testing matrix.