Update DateTimeFormat proleptic Gregorian testing to include era (#3722)

Ref tc39/ecma402#723

Author: gibson042

test/intl402/DateTimeFormat/prototype/format/proleptic-gregorian-calendar.js

@@ -15,7 +15,7 @@ var dates = [
     -8640000000000000 // beginning of ECMAScript time
 ];
 
-var format = new Intl.DateTimeFormat(["en-US"], {year: "numeric", month: "long", timeZone: "UTC"});
+var format = new Intl.DateTimeFormat(["en-US"], {year: "numeric", era: "short", timeZone: "UTC"});
 
 // this test requires a Gregorian calendar, which we usually find in the US
 assert.sameValue(format.resolvedOptions().calendar, "gregory", "Internal error: Didn't find Gregorian calendar");
@@ -24,6 +24,8 @@ dates.forEach(function (date) {
     var year = new Date(date).getUTCFullYear();
     var expectedYear = year <= 0 ? 1 - year : year;
     var expectedYearString = expectedYear.toLocaleString(["en-US"], {useGrouping: false});
+    var expectedEra = year <= 0 ? /BC/ : /AD|(?:^|[^B])CE/;
     var dateString = format.format(date);
     assert.notSameValue(dateString.indexOf(expectedYearString), -1, "Formatted year doesn't contain expected year – expected " + expectedYearString + ", got " + dateString + ".");
+    assert(expectedEra.test(dateString), "Formatted year doesn't contain expected era – expected " + expectedEra + ", got " + dateString + ".");
 });