ICU-22954 disable TestCharString on Cygwin

[markusicu]

See ICU-22999 for details

Disabled TestCharString.
Then TestCharStrAppendNumber crashed, so disabled that too.

Checklist

• Required: Issue filed: ICU-22954
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• 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

[roubert]

As the problem (whatever it might be) isn't in these tests, it just happens to manifest there, this shouldn't be merged.