Restore json to alt_json conversion in unit test

A unit test in unit-alt_string.cpp converts a json object to an alt_json
object. During the json_pointer refactor this failed to compile and was
commented out.
The failure can no longer be reproduced with the current code.

This commit restores the test to its originally proposed form.

Closes nlohmann#3425.

test/src/unit-alt-string.cpp

@@ -328,15 +328,11 @@ TEST_CASE("alternative string type")
 
     SECTION("JSON pointer")
     {
-        // conversion from json to alt_json fails to compile (see #3425);
-        // attempted fix(*) produces: [[['b','a','r'],['b','a','z']]] (with each char being an integer)
-        // (*) disable implicit conversion for json_refs of any basic_json type
-        // alt_json j = R"(
-        // {
-        //     "foo": ["bar", "baz"]
-        // }
-        // )"_json;
-        auto j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
+        alt_json j = R"(
+        {
+            "foo": ["bar", "baz"]
+        }
+        )"_json;
 
         CHECK(j.at(alt_json::json_pointer("/foo/0")) == j["foo"][0]);
         CHECK(j.at(alt_json::json_pointer("/foo/1")) == j["foo"][1]);