ICU-22908 MF2: Update spec tests and update implementation for recent spec changes

[catamorphism]

Updating the spec tests requires two implementation changes:

• Match on variables rather than expressions -- see https://github.com/unicode-org/message-format-wg/pull/877
• Require attribute values to be literals (if present) -- see https://github.com/unicode-org/message-format-wg/pull/894

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22908
• [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