feat: mod test (#414)

- Adds the mod-test block snippet
- Updates the `match` snippet to be more general

snippets/rust/rust.json

@@ -391,6 +391,19 @@
         ],
         "description": "struct … { … }"
     },
+    "modtest": {
+        "prefix": "modtest",
+        "body": [
+            "#[cfg(test)]",
+            "mod test {",
+            "    #[test]",
+            "    fn ${1:name}() {",
+            "        ${2:todo!();}",
+            "    }",
+            "}"
+        ],
+        "description": "#[cfg(test)]\nmod test {...}"
+    },
     "test": {
         "prefix": "test",
         "body": [