Add table of errors to test README (#706)

[catamorphism]

Update README for tests directory to include a table relating error names in the test schema to error names in the spec.

[aphillips]

Minor issues. Thanks for making this PR.

[aphillips]

I can imagine writing tests for my own formatting functions that might use such an error. Would we be better off to define the schema name and then say:

Suggested change

	The "Message Function Error" error name used in the spec
	is not included in the schema,
	as it is intended to be an umbrella category
	for implementation-specific errors.
	> [!NOTE]
	> The error name `message-function-error` does not appear in any of the
	> tests found in this test suite because the "Message Function Error"
	> category of errors is intended as an umbrella category
	> for implementation-specific errors.

[eemeli]

The current schema is designed to specifically support our own tests, and it's not really extensible e.g. due to incorporating "additionalProperties": false requirements in many places. This means that adding message-function-error is beyond its current intended use cases.

As JSON Schema in general is not extensible (it has no concept of inheritance, for instance), other test suites that would have such a need probably need to copy our schema and patch it locally, or not rely on an explicit JSON Schema document.

I found the PR's proposed language appropriate.