Consider helpers for `formatToParts` assertions

[gibson042]

(originally posted by @gibson042 in #4447 (comment))

I wonder if a harness file should define some formatToParts assertion helpers:

function assertFormatPart(part, type, value, label) {
  var suffix = label ? ', ' + label : '';
  assert.sameValue(part.type, type, 'part type' + suffix);
  if (value === undefined) return;
  assert.sameValue(part.value, value, 'part value' + suffix);
}

function assertFormatParts(parts, expectations, label) {
  var suffix = label ? ', ' + label : '';
  assert.sameValue(parts.length, expectations.length, 'parts count' + suffix);
  for (var i = 0; i < parts.length; i++) {
    assertFormatPart(parts[i], expectations[i][0], expectations[i][1],
      suffix && (suffix + ' part ' + i));
  }
}

for use in files like test/intl402/DateTimeFormat/prototype/formatToParts/dayPeriod-long-en.js:

for (var h = 0; h < 24; h++) {
  var parts = long.formatToParts(inputs[h]);
  assertFormatParts(parts, [['dayPeriod']], 'hour ' + h + ' long dayPeriod');
  …
  var partsNumeric = longNumeric.formatToParts(inputs[h]);
  assertFormatParts(partsNumeric,
    [['hour', String((h % 12) || 12)], ['literal'], ['dayPeriod']],
    'numeric hour ' + h + ' must precede long dayPeriod');

  counts[i]++;
  prevDayPeriod = dayPeriod;
}

Similar functions already exist in various files, with names like

• assertParts
• assertSameParts
• compare
• compareFormatToPartsSnapshot
• formatAsString
• partsEquals
• reduce
• serializeTypesAndLiteral
• verifyFormatParts
• etc.