Temporal: Test coverage for rejecting too large dates in PlainMonthDa…

…y strings

This adds coverage for the normative change in
tc39/proposal-temporal#3054, which specifies that
implementations must throw on out-of-range dates in RFC 9557 strings for
non-ISO calendars.

harness/temporalHelpers.js

@@ -1105,6 +1105,10 @@ var TemporalHelpers = {
         "11-18[U-CA=iso8601]",
         "11-18[u-CA=iso8601]",
         "11-18[FOO=bar]",
+        "-999999-01-01[u-ca=gregory]",
+        "-999999-01-01[u-ca=chinese]",
+        "+999999-01-01[u-ca=gregory]",
+        "+999999-01-01[u-ca=chinese]",
       ];
     },
 
@@ -1136,6 +1140,10 @@ var TemporalHelpers = {
         "1976-10-01",
         "--10-01",
         "--1001",
+        "-999999-10-01",
+        "-999999-10-01[u-ca=iso8601]",
+        "+999999-10-01",
+        "+999999-10-01[u-ca=iso8601]",
       ];
     },