Add todo spec test for libsass issue 2020

sass/libsass#2020

Author: mgreter

spec/libsass-todo-issues/issue_2020/expected_output.css

@@ -0,0 +1,3 @@
+form {
+  sel: form;
+  sel: form; }

spec/libsass-todo-issues/issue_2020/input.scss

@@ -0,0 +1,6 @@
+form {
+  $selector: nth(&, 1);
+  sel: inspect($selector);
+  $selector: nth($selector, 1);
+  sel: inspect($selector);
+}