gopls/internal/test/marker: add a folding ranges test case

This CL adds a test case for folding_range, and demonstrates gopls
current reports folding range correctly. So the issue may not caused by gopls.

Updates: golang/go#73735

Change-Id: I60e7a649ee10a4ffecb5468981e80c96f0ada455
Reviewed-on: https://go-review.googlesource.com/c/tools/+/673395
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: xieyuschen

gopls/internal/test/marker/testdata/foldingrange/a.txt

@@ -131,6 +131,11 @@ func _(
 	c int,
 ) {
 }
+
+func _() { // comment
+
+}
+
 -- @raw --
 package folding //@foldingrange(raw)
 
@@ -262,3 +267,8 @@ func _(<52 kind="">
 	c int,
 </52>) {<53 kind="">
 </53>}
+
+func _() {<54 kind=""> // comment
+
+</54>}
+