add tests

Author: ota-meshi

tests/fixtures/parser/ast/$var-input.svelte

@@ -0,0 +1,4 @@
+<script>
+  import { a } from "./stores"
+  $a = 42
+</script>

tests/fixtures/parser/ast/$var-no-unused-vars-result.json

@@ -0,0 +1,8 @@
+[
+  {
+    "ruleId": "no-unused-vars",
+    "code": "a",
+    "line": 2,
+    "column": 12
+  }
+]