Only opt-into tests for TOML

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

test/SerialIOTest.cpp

@@ -85,7 +85,8 @@ std::vector<std::string> testedFileExtensions()
         allExtensions.begin(), allExtensions.end(), [](std::string const &ext) {
             // sst and ssc need a receiver for testing
             // bp4 is already tested via bp
-            return ext == "sst" || ext == "ssc" || ext == "bp4";
+            return ext == "sst" || ext == "ssc" || ext == "bp4" ||
+                ext == "toml";
         });
     return {allExtensions.begin(), newEnd};
 }
@@ -1622,16 +1623,13 @@ TEST_CASE("dtype_test", "[serial]")
             dtype_test(t);
             dtype_test(t, R"(json.mode = "template")");
         }
-        else if (t == "toml")
-        {
-            dtype_test(t);
-            dtype_test(t, R"(toml.mode = "template")");
-        }
         else
         {
             dtype_test(t);
         }
     }
+    dtype_test("toml");
+    dtype_test("toml", R"(toml.mode = "template")");
 }
 
 inline void write_test(const std::string &backend)