Add a JSON test suite (#604)

* Move test/* -> test/xml/

* Add test/json/ as a copy of packages/mf2-messageformat/src/__fixtures/ from messageformat/messageformat@11c95da

* Add test/json/README.md

* Apply suggestions from code review

Co-authored-by: Addison Phillips <[email protected]>

* Drop the previous XML tests, move test/json/* to just test/

---------

Co-authored-by: Addison Phillips <[email protected]>

Author: eemeli

test/README.md

@@ -0,0 +1,28 @@
+The files in this directory were originally copied from the [messageformat project](https://github.com/messageformat/messageformat/tree/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/__fixtures)
+and are here relicensed by their original author (Eemeli Aro) under the Unicode License.
+
+These test files are intended to be useful for testing multiple different message processors in different ways:
+
+- `syntax-errors.json` — An array of strings that should produce a Syntax Error or a Data Model Error when parsed.
+
+- `test-core.json` — An array of test cases that do not depend on any registry definitions.
+  Each test may include some of the following fields:
+  - `src: string` (required) — The MF2 syntax source.
+  - `exp: string` (required) — The expected result of formatting the message to a string.
+  - `locale: string` — The locale to use for formatting. Defaults to 'en-US'.
+  - `params: Record<string, string | number | null | undefined>` — Parameters to pass in to the formatter for resolving external variables.
+  - `parts: object[]` — The expected result of formatting the message to parts.
+  - `cleanSrc: string` — A normalixed form of `src`, for testing stringifiers.
+  - `errors: { type: string }[]` — The runtime errors expected to be emitted when formatting the message.
+  - `only: boolean` — Normally not set. A flag to use during development to only run one or more specific tests.
+
+- `test-function.json` — An object with string keys and arrays of test cases as values,
+  using the same definition as for `test-core.json`.
+  The keys each correspond to a function that is used in the tests.
+
+TypeScript `.d.ts` files are included for `test-core.json` and `test-function.json` with the above definition.
+
+Some examples of test harnesses using these tests, from the source repository:
+- [CST parse/stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/cst/cst.test.ts)
+- [Data model stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/data-model/stringify.test.ts)
+- [Formatting tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/messageformat.test.ts)