add a test

Author: cometkim

test/utils.js

@@ -9,6 +9,9 @@ import {
   isSIP,
   isTIP,
   isSSP,
+  isHighSurrogate,
+  isLowSurrogate,
+  surrogatePairToCodePoint,
 } from 'unicode-segmenter/utils';
 
 fc.configureGlobal({
@@ -17,6 +20,25 @@ fc.configureGlobal({
   numRuns: 100_000,
 });
 
+test('surrogate pairs', () => {
+  fc.assert(
+    fc.property(
+      fc.integer({ min: 0xffff + 1, max: 0xeffff }),
+      // @ts-ignore
+      cp => {
+        let ch = String.fromCodePoint(cp);
+        let hi = ch.charCodeAt(0);
+        let lo = ch.charCodeAt(1);
+        return (
+          isHighSurrogate(hi) &&
+          isLowSurrogate(lo) &&
+          cp === surrogatePairToCodePoint(hi, lo)
+        );
+      }
+    )
+  )
+});
+
 test('isBMP', () => {
   fc.assert(
     fc.property(