ICU-22104 Add tests for CPP alphabetic index set/get maximum label count

[sven-oly]

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22104
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• [x] Tests included, if applicable
• [x] API docs and/or User Guide docs changed or added, if applicable

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