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

[catamorphism]

Note: this PR updates the shared tests, but doesn't update the ICU4J implementation with the two spec changes, so ICU4J tests will fail in CI for now.

I'll check with @mihnita about the Java side.

[mihnita]

We also have this new fresh issue filed: https://unicode-org.atlassian.net/browse/ICU-22906

I am almost tempted to separate the tests again...

[mihnita]

I am tempted to say "let's drop it"

We were supposed to be code freeze, and this is something that my superstition feels very strongly: submit on Friday evening :-)

Even if it lands on Monday, it is still "Friday afternoon" with respect to the ICU release schedule.

[jira-pull-request-webhook[bot]]

Notice: the branch changed across the force-push!

• icu4c/source/test/intltest/messageformat2test_read_json.cpp is different

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[jira-pull-request-webhook[bot]]

Notice: the branch changed across the force-push!

• icu4c/source/i18n/messageformat2_checker.cpp is different
• icu4c/source/i18n/messageformat2_checker.h is different
• icu4c/source/i18n/messageformat2_data_model.cpp is different
• icu4c/source/i18n/messageformat2_parser.cpp is different
• icu4c/source/i18n/messageformat2_parser.h is different
• icu4c/source/i18n/messageformat2_serializer.cpp is different
• icu4c/source/i18n/messageformat2.cpp is different
• icu4c/source/i18n/unicode/messageformat2_data_model.h is different
• icu4c/source/i18n/unicode/messageformat2.h is different
• icu4c/source/test/intltest/messageformat2test_custom.cpp is different
• icu4c/source/test/intltest/messageformat2test_read_json.cpp is different
• icu4c/source/test/intltest/messageformat2test_utils.h is different
• icu4c/source/test/intltest/messageformat2test.cpp is different
• testdata/message2/normalization.json is now changed in the branch

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