Add tests for tl::unexpected guide

tests/issues.cpp

@@ -159,4 +159,11 @@ TEST_CASE("Issue 122", "[issues.122]") {
      tl::expected<move_tracker, int> res;
      res.emplace(); // why moved?
      REQUIRE(res.value().moved == 0);
-}
+}
+
+#ifdef __cpp_deduction_guides
+TEST_CASE("Issue 89", "[issues.89]") { 
+    auto s = tl::unexpected("Some string");
+    REQUIRE(s.value() == "Some string");
+}
+#endif