fix: Adapt combobox unit tests to FluentUI internal change

[marek-mihok]

The PR fulfills these requirements: (check all the apply)

• It's submitted to the main branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• If changes were made to ui folder, unit tests (make test) still pass.
• New/updated tests are included

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In updated version of FluentUI there is no longer the span with a title attribute, but span with a text content instead.

<span class="ms-ComboBox-optionText css-158">Cyan</span>