Add todo spec test for libsass issue 2808

sass/libsass#2808

Author: mgreter

spec/libsass-todo-issues/issue_2808.hrx

@@ -0,0 +1,9 @@
+<===> input.scss
+test {
+  content: str-slice(abcdef, -10, 2)
+}
+
+<===> output.css
+test {
+  content: ab;
+}