Change logic behavior and add test

Author: Enkidu93

src/SIL.Machine/Corpora/UpdateUsfmParserHandler.cs

@@ -388,14 +388,18 @@ public string GetUsfm(UsfmStylesheet stylesheet)
 
                 if (tokens.Count > 0 && tokens[0].Marker == "id")
                 {
+                    int index = 1;
                     if (tokens.Count > 1 && tokens[1].Type == UsfmTokenType.Text)
                     {
-                        tokens.InsertRange(2, remarkTokens);
+                        index = 2;
                     }
-                    else
+                    while (tokens[index].Marker == "rem")
                     {
-                        tokens.InsertRange(1, remarkTokens);
+                        index++;
+                        if (tokens.Count > index && tokens[index].Type == UsfmTokenType.Text)
+                            index++;
                     }
+                    tokens.InsertRange(index, remarkTokens);
                 }
             }
             return tokenizer.Detokenize(tokens);

tests/SIL.Machine.Tests/Corpora/UpdateUsfmParserHandlerTests.cs

@@ -1219,6 +1219,7 @@ public void GetUsfm_PreferExisting_AddRemark()
         };
         string usfm =
             @"\id MAT - Test
+\rem Existing remark
 \c 1
 \v 1 Some text
 \v 2
@@ -1232,11 +1233,31 @@ public void GetUsfm_PreferExisting_AddRemark()
         );
         string result =
             @"\id MAT - Test
+\rem Existing remark
 \rem New remark
 \c 1
 \v 1 Some text
 \v 2 Update 2
 \v 3 Other text
+";
+
+        AssertUsfmEquals(target, result);
+
+        target = UpdateUsfm(
+            rows,
+            target,
+            textBehavior: UpdateUsfmTextBehavior.PreferExisting,
+            remarks: ["New remark 2"]
+        );
+        result =
+            @"\id MAT - Test
+\rem Existing remark
+\rem New remark
+\rem New remark 2
+\c 1
+\v 1 Some text
+\v 2 Update 2
+\v 3 Other text
 ";
 
         AssertUsfmEquals(target, result);