Only opt-into tests for TOML

Has the same implementation as JSON anyway, and it makes tests run into
timeouts otherwise.

Author: franzpoeschel

examples/14_toml_template.cpp

@@ -85,7 +85,8 @@ void read()
 {
   "iteration_encoding": "variable_based",
   "json": {
-    "mode": "template"
+    "dataset": {"mode": "template"},
+    "attribute": {"mode": "short"}
   }
 }
 )";

test/SerialIOTest.cpp

@@ -1646,7 +1646,15 @@ TEST_CASE("dtype_test", "[serial]")
 {
     for (auto const &t : testedFileExtensions())
     {
-        dtype_test(t);
+        if (t == "json")
+        {
+            dtype_test(t);
+            dtype_test(t, R"(json.mode = "template")");
+        }
+        else
+        {
+            dtype_test(t);
+        }
     }
     dtype_test("json", R"(toml.dataset.mode = "template")");
     if (auto extensions = getFileExtensions();