Add a test for removing nested parens

Author: topecongiro

tests/source/paren.rs

@@ -0,0 +1,8 @@
+// Remove nested parens.
+
+fn main() {
+    let x = (((1)));
+    let y = (/* comment */((2)));
+    let z = (((3)/* comment */));
+    let a = (((4/* comment */)));
+}

tests/target/paren.rs

@@ -0,0 +1,8 @@
+// Remove nested parens.
+
+fn main() {
+    let x = (1);
+    let y = (/* comment */(2));
+    let z = ((3)/* comment */);
+    let a = (4/* comment */);
+}