test: expected-output fixtures for JSON inputs (salvaged from #2509)

[schani]

This salvages the surviving half of #2509 by @Adam724: the ability to assert what a fixture's output should be when it legitimately differs from the input.

Problem

The JSON fixture harness asserts round-trip output ≈ input, with unconditional leniency: extra null properties in the output are always allowed, and allowMissingNull languages may drop nulls. That makes correct behaviors where the output should differ from the input untestable. The concrete case from #2509: Go's omitempty should drop null/empty fields, and nothing could assert that it does.

Mechanism

This ports the JSON Schema fixtures' existing .out.<feature>.json convention to JSON inputs, instead of #2509's parallel path-keyed option map:

• A JSON input foo.json may come with an expected-output file foo.out.<key>.json next to it.
• The file applies to a run when <key> is one of the language's features, or the name of a renderer option that this particular run sets (via a pinned quickTestRendererOptions entry).
• When it applies, the run's output must match the expected file strictly — none of the usual null leniency. When absent, behavior is unchanged.
• .out. files are excluded from test-input enumeration.

Test case

test/inputs/json/priority/omit-empty.json is #2509's test input (a null field that omitempty drops):

{ "results": [ { "age": 52, "name": null }, { "age": 27 } ] }

A pinned quickTestRendererOptions entry runs it for Go with omit-empty, and omit-empty.out.omit-empty.json asserts the null field is actually dropped:

{ "results": [ { "age": 52 }, { "age": 27 } ] }

The plain (no-option) runs of this input still use the ordinary lenient round-trip comparison in every language, including Go: by default the Go renderer only tags non-null/union/any optionals with ,omitempty, so the null survives the round trip there.

The input lives in priority/, not misc/, so it runs in CI — QUICKTEST skips misc/, which is why #2509's version of this test never actually ran in CI.

Relation to #2509

• Superseded half: Enhance testing capabilities #2509's per-test CLI options mechanism (test/lib/optionMap.ts) has since been covered by quickTestRendererOptions in test/languages.ts, so it is not ported.
• Salvaged half: the expected-output (*.in.json / *.out.json) idea, redesigned onto the existing .out.<feature>.json convention. The test input and expected output are @Adam724's, credited as co-author.

Testing

• FIXTURE=golang script/test (full), FIXTURE=python script/test, FIXTURE=typescript script/test, FIXTURE=schema-golang script/test all green.
• Negative check: with a stale expected file that keeps the null, the pinned Go run fails and the plain runs pass, confirming the strict comparison engages exactly where the expected file applies.

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