Add pseudo-mod TEST_DATA folder and auto-load it

data/mods/TEST_DATA/README.md

@@ -0,0 +1,11 @@
+# Test Data pseudo-mod #
+
+This mod is purely for loading data to be used by `tests/cata_test`. It is
+automatically loaded by `tests/test_main.cpp`, so any items, recipes, or other
+content defined in the mod will be available to everything in `tests/`.
+
+The benefit of using this mod for test data is that it allows a clean
+separation of tests from in-game content. Instead of testing with content in
+the main `data/json` directory, functional tests can use `TEST_DATA` content
+to ensure a more stable and controllable set of example data.
+

data/mods/TEST_DATA/modinfo.json

@@ -0,0 +1,12 @@
+[
+  {
+    "type": "MOD_INFO",
+    "ident": "test_data",
+    "name": "TESTING DATA",
+    "description": "Adds mockup items, recipes, and other content for use by automated tests.",
+    "category": "content",
+    "//": "Not really obsolete! Marked as such to prevent it from showing in the main list",
+    "obsolete": true,
+    "dependencies": [ "dda" ]
+  }
+]

tests/test_main.cpp

@@ -78,6 +78,9 @@ static std::vector<mod_id> extract_mod_selection( std::vector<const char *> &arg
             ret.emplace_back( mod_name );
         }
     }
+    // Always load test data mod
+    ret.emplace_back( "test_data" );
+
     return ret;
 }