ICU-22794 MF2: De-duplicate C++ and Java test data

[catamorphism]

This PR moves all the .json files for data-driven ICU4C and ICU4J tests into a single top-level directory, icu/testdata/.

I did my best to remove duplicate tests.

This involved fixing several ICU4J parser bugs, almost all involving whitespace handling.

Some tests still had to be ignored in either Java or C++, in which cases I filed ICU tickets.

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22794
• [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