vue-test-utils: could not overwrite property $route, this is usually caused by a plugin that has added the property as a read-only value

So this is a bug with vue-test-utils. If you are using VueRouter anywhere (even if it's not used in any unit test), you will get the above error.

A work around is to use process.env.NODE_ENV in your unit tests and set it to 'test', and wherever you're using VueRouter, check process.env.NODE_ENV like so:

if (!process || process.env.NODE_ENV !== 'test') {
    Vue.use(VueRouter)
}

at least until vue-test-utils bug is fixed, this should fix this problem

I think these docs are relevant to your situation:

Common gotchas

Installing Vue Router adds $route and $router as read-only properties on Vue prototype.

This means any future tests that try to mock $route or $router will fail.

To avoid this, never install Vue Router globally when you're running tests; use a localVue as detailed above.

The error you're seeing indicates that one of your components (and outside your test code) is installing VueRouter (e.g., Vue.use(VueRouter)).

To address the issue, search for the VueRouter installation in your component code path, including any of their imports, and refactor it so that the import is not required there. Typically, the installation of VueRouter exists only in main.js or its imports.

GitHub demo