test: #3630425 Add test coverage for the operations column header

Chamil Snjeewarequested to merge
issue/drupal-3630425:main into main

The multi-value field widget table had an empty th for the actions column. Empty header cells are an accessibility problem for screen reader users.

The header cell now contains a visually hidden "Operations" label.

Tests: FormTest::testFieldFormUnlimited() now checks two things. The table header has no empty th. The header contains the visually hidden "Operations" text. The test fails without the fix.

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