Enhancement of validation.

[vdbergh]

Many data structure in Fishtest contain references to a run objects. Naively validating such data structures means that we are also validating the embedded run objects, which is wasteful as these are already validated elsewhere.

The latest version of vtjson makes it possible to change subschemas during validation.

In this PR we define the cache_schema as

cache_schema = {
    run_id: {
        "run": set_label(runs_schema, "runs_schema"),
    "is_changed": bool,
        "last_sync_time": ufloat,
        "last_access_time": ufloat,
    },
}

and we perform validation via

validate(cache_schema, self.run_cache, name="run_cache", subs={"runs_schema": dict})

Requires upgrade of vtjson.

[vdbergh]

Part of the program to document and validate the data structures used by Fishtest...

[ppigazzini]

PROD rebuilt from scratch, thank you @vdbergh