ICU-22954 Workaround to make the icu4c-windows-cygwin-gcc CI job work

[roubert]

For some unknown reason the use of std::u16string in class IcuTestErrorCode since commit 5dff777 causes the icu4c-windows-cygwin-gcc CI job to fail.

Checklist

• Required: Issue filed: ICU-22954
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

ALLOW_MANY_COMMITS=true

[markusicu]

thanks!