generalize folding tests

By using xml-like tags, we will be able to test nested foldings.

crates/ra_editor/src/folding_ranges.rs

@@ -170,7 +170,7 @@ mod tests {
     use test_utils::extract_ranges;
 
     fn do_check(text: &str, fold_kinds: &[FoldKind]) {
-        let (ranges, text) = extract_ranges(text);
+        let (ranges, text) = extract_ranges(text, "fold");
         let file = SourceFileNode::parse(&text);
         let folds = folding_ranges(&file);
 
@@ -198,22 +198,22 @@ mod tests {
     #[test]
     fn test_fold_comments() {
         let text = r#"
-<|>// Hello
+<fold>// Hello
 // this is a multiline
 // comment
-//<|>
+//</fold>
 
 // But this is not
 
 fn main() {
-    <|>// We should
+    <fold>// We should
     // also
     // fold
-    // this one.<|>
-    <|>//! But this one is different
-    //! because it has another flavor<|>
-    <|>/* As does this
-    multiline comment */<|>
+    // this one.</fold>
+    <fold>//! But this one is different
+    //! because it has another flavor</fold>
+    <fold>/* As does this
+    multiline comment */</fold>
 }"#;
 
         let fold_kinds = &[
@@ -228,11 +228,11 @@ fn main() {
     #[test]
     fn test_fold_imports() {
         let text = r#"
-<|>use std::{
+<fold>use std::{
     str,
     vec,
     io as iop
-};<|>
+};</fold>
 
 fn main() {
 }"#;
@@ -244,12 +244,12 @@ fn main() {
     #[test]
     fn test_fold_import_groups() {
         let text = r#"
-<|>use std::str;
+<fold>use std::str;
 use std::vec;
-use std::io as iop;<|>
+use std::io as iop;</fold>
 
-<|>use std::mem;
-use std::f64;<|>
+<fold>use std::mem;
+use std::f64;</fold>
 
 use std::collections::HashMap;
 // Some random comment
@@ -265,17 +265,17 @@ fn main() {
     #[test]
     fn test_fold_import_and_groups() {
         let text = r#"
-<|>use std::str;
+<fold>use std::str;
 use std::vec;
-use std::io as iop;<|>
+use std::io as iop;</fold>
 
-<|>use std::mem;
-use std::f64;<|>
+<fold>use std::mem;
+use std::f64;</fold>
 
-<|>use std::collections::{
+<fold>use std::collections::{
     HashMap,
     VecDeque,
-};<|>
+};</fold>
 // Some random comment
 
 fn main() {