Add more exhaustive syntax tests (#843)

[catamorphism]

Beginning to address #843 and the corresponding item under https://github.com/unicode-org/message-format-wg/wiki/Things-That-Need-Doing .

I tried to test each variation of each rule (up to but not including identifier.) I didn't try to test combinations of variations. For example, for this rule:

complex-message   = *(declaration [s]) complex-body [s]

the idea would be to add tests that exercise the following four (partial) derivations:

complex-message -> complex-body
complex-message -> complex-body s
complex-message -> declaration complex-body
complex-message -> declaration s complex-body

This isn't exhaustive by any means, but it was broad enough that I found one bug in the ICU4J parser (the ".input{$x}{{}} test case) and two bugs in the ICU4C parser (the \\\\ test case and the { #a} test case).

[catamorphism]

The validation CI check is failing because I used the comment field, which I added to the version of the schema in the conformance repo -- I guess I'll go ahead and update that in the same PR. Actually, I'm not sure which repo should be the source of truth. Any opinions?

[eemeli]

Tests may already include a "description", why not use that?

message-format-wg/test/schemas/v0/tests.schema.json

Lines 135 to 142 in 4514e88

	"test": {
	"type": "object",
	"additionalProperties": false,
	"properties": {
	"description": {
	"type": "string",
	"description": "Information about the test."
	},

The spec tests in this repo need to match the schema that's also in this repo.

[catamorphism]

Tests may already include a "description", why not use that?

Ah, didn't notice that. Fixed.