Add todo spec test for libsass issue 2017

sass/libsass#2017

Author: mgreter

spec/libsass-todo-issues/issue_2017/expected_output.css

@@ -0,0 +1,7 @@
+foo {
+  bar: baz; }
+
+foo a:not(.btn):not(foo) {
+  color: red; }
+foo a:not(.btn):not(foo) {
+  color: red; }

spec/libsass-todo-issues/issue_2017/input.scss

@@ -0,0 +1,15 @@
+foo {
+  bar: baz;
+}
+
+@mixin link() {
+  a:not(.btn) {
+    color: red;
+  }
+}
+
+foo {
+  @include link;
+  @extend .btn;
+  @include link;
+}