Cherry pick some loader string test cases (#2050)

Author: dcodeIO

lib/loader/tests/index.js

@@ -59,6 +59,22 @@ function test(file) {
     assert.strictEqual(exports.strlen(ref), str.length);
   }
 
+  // should be able to allocate and work with a string containing an isolated high surrogate
+  {
+    let str = "𝄞".substring(0, 1);
+    let ref = exports.__newString(str);
+    assert.strictEqual(exports.__getString(ref), str);
+    assert.strictEqual(exports.strlen(ref), str.length);
+  }
+
+  // should be able to allocate and work with a string containing an isolated low surrogate
+  {
+    let str = "𝄞".substring(1);
+    let ref = exports.__newString(str);
+    assert.strictEqual(exports.__getString(ref), str);
+    assert.strictEqual(exports.strlen(ref), str.length);
+  }
+
   // should be able to allocate a typed array
   {
     let arr = [1, 2, 3, 4, 5, 0x80000000 | 0];