Factor out custom stringify function

[smellman]

Describe the bug
Same as maplibre/maplibre-gl-js#967
In test/lib/utils.ts we have a stringify and the following comment:

import compactStringify from 'json-stringify-pretty-compact';

// we have to handle this edge case here because we have test fixtures for this
// edge case, and we don't want UPDATE=1 to mess with them
export function stringify(v) {
    let s = compactStringify(v);

    if (s.indexOf('\u2028') >= 0) {
        s = s.replace(/\u2028/g, '\\u2028');
    }
    if (s.indexOf('\u2029') >= 0) {
        s = s.replace(/\u2029/g, '\\u2029');
    }
    return s;
}

maplibre-gl-js has removed all this function but maplibre-style-spec still available.
If this implementation is needed, close this issue.
Note: Also maplibre-style-spec uses stringifyPretty from json-stringify-pretty-compact.