Update testdata/message2/README.txt

Author: catamorphism

Diff for: testdata/message2/README.txt

@@ -1,19 +1,14 @@
 © 2024 and later: Unicode, Inc. and others.
 License & terms of use: http://www.unicode.org/copyright.html
 
-The format of the JSON files in this directory follows the same format as `test-core.json`
-in the spec, described in:
+The format of the JSON files in this directory and subdirectories
+follow the test schema defined in the Conformance repository:
 
-https://github.com/unicode-org/message-format-wg/blob/main/test/README.md
+https://github.com/unicode-org/conformance/blob/main/schema/message_fmt2/testgen_schema.json
 
-The `parts` field is not used.
+(as of https://github.com/unicode-org/conformance/pull/255 or later).
 
-# JSON extensions
-
-An additional `comment` field may be present, which is only for human readers.
-
-A "srcs" field, whose value is an array of strings, may be present instead
-of "src". The strings are concatenated to get the message.
+# JSON notes
 
 In the "params" field, a date parameter can be expressed as:
 { "date": n }
@@ -46,43 +41,3 @@ If present, "char" reflects the expected character offset and "line"
 reflects the expected line number in the parse error.
 The files with "diagnostics" in the name have these fields filled in.
 
-# ICU4C vs. ICU4J tests
-
-The following tests are run in both ICU4C and ICU4J:
-
-* alias-selector-annotations.json
-* duplicate-declarations.json
-* icu-parser-tests.json
-  - Two tests removed while single-sourcing tests, because a `{{}}` message body
-  had to be added to get it to parse in ICU4C, and this broke the test in ICU4J.
-  These tests are in icu-parser-tests-old.json
-* icu-test-functions.json
-  - Some tests marked as ignored
-* icu-test-previous-release.json
-  - Some tests marked as ignored
-* icu-test-selectors.json
-* markup.json
-* matches-whitespace.json
-  - Some tests marked as ignored
-* more-data-model-errors.json
-* more-syntax-errors.json
-* reserved-syntax.json
-  - All tests marked as ignored in Java (resolution errors are suppressed)
-* resolution-errors.json
-  - All tests marked as ignored in Java (resolution errors are suppressed)
-* runtime-errors.json
-  - All tests marked as ignored in Java (message function errors are suppressed)
-* syntax-errors-diagnostics.json
-* tricky-declarations.json
-* valid-tests.json
-  - Some tests marked as ignored
-* spec/*
-  - Some tests in test-core.json and test-functions.json marked as ignored
-
-The following tests are only run in ICU4C, either because ICU4J doesn't check
-for invalid options, or because ICU4J doesn't report line/column numbers for
-parse errors:
-* invalid-number-literals-diagnostics.json
-* invalid-options.json
-* syntax-errors-diagnostics-multiline.json
-* syntax-errors-end-of-input.json