refactor: optimize rule tester implementation

tests/src/rule-tester.js

@@ -25,35 +25,22 @@ class FlatCompatRuleTester {
       return config;
     }
 
-    const obj = {
-      languageOptions: { parserOptions: {} },
+    const { parser, parserOptions = {}, languageOptions = {}, ...remainingConfig  } = config;
+    const { ecmaVersion, sourceType, ...remainingParserOptions } = parserOptions;
+    const parserObj = typeof parser === 'string' ? require(parser) : parser;
+
+    return {
+      ...remainingConfig,
+      languageOptions: {
+        ...languageOptions,
+        ...parserObj ? { parser: parserObj } : {},
+        ...ecmaVersion ? { ecmaVersion } : {},
+        ...sourceType ? { sourceType } : {},
+        parserOptions: {
+          ...remainingParserOptions,
+        },
+      },
     };
-
-    for (const [key, value] of Object.entries(config)) {
-      if (key === 'parser') {
-        obj.languageOptions.parser = typeof value === 'string' ? require(value) : value;
-
-        continue;
-      }
-
-      if (key === 'parserOptions') {
-        for (const [option, val] of Object.entries(value)) {
-          if (option === 'ecmaVersion' || option === 'sourceType') {
-            obj.languageOptions[option] = val;
-
-            continue;
-          }
-
-          obj.languageOptions.parserOptions[option] = val;
-        }
-
-        continue;
-      }
-
-      obj[key] = value;
-    }
-
-    return obj;
   }
 }