add test for GetOrAddFirstChild

Author: mikeebowen

test/DocumentFormat.OpenXml.Tests/ofapiTest/OpenXmlElementTest2.cs

@@ -216,5 +216,20 @@ internal override void ConfigureMetadata(ElementMetadata.Builder builder)
         private class ChildElement : OpenXmlLeafElement
         {
         }
+
+        /// <summary>
+        /// A test for OpenXmlElement.GetOrAddFirstChild.
+        /// </summary>
+        [Fact]
+        public void GetOrAddFirstChildTest()
+        {
+            Paragraph p = new();
+            Run r = p.GetOrAddFirstChild<Run>();
+            Assert.NotNull(r);
+            Assert.Same(r, p.GetFirstChild<Run>());
+
+            var r2 = p.GetOrAddFirstChild<Run>();
+            Assert.Same(r, r2);
+        }
     }
 }