test: don't test for prefer-const in .js files

Previously, the `prefer-const` rule is from
'@typescript-eslint/recommended`, which is enabled for all files;

In @typescript-eslint v3, '@typescript-eslint/recommended` inherits this
rule by extending from `@typescript-eslint/eslint-recommended`, which,
only applies to `.ts/.tsx` files

Author: haoqunjiang

test/index.spec.js

@@ -49,6 +49,5 @@ test('should not override eslint:recommended rules for normal js files', async (
   expect(stdout).toMatch('no-const-assign')
 
   // errors in .js file
-  expect(stdout).toMatch('prefer-const')
   expect(stdout).toMatch('no-undef')
 })