[router] remove duplicate context mocking function

[marklawlor]

Why

There are two functions used to mocking a context in our tests, with createMockContextModule only being used by the configFromFs.

Unfortunately the tests using configFromFs are subtlety incorrect. context.keys() are relative paths (with a ./ prefix). Without this, getRoutes() creates a two tier state, with an empty name route for the top level { name: '' }. createMockContextModule does not enforce this, so while technically the getPathsFromState tests are correct, what they are testing and what our expectation is are different.

You can verify that these tests were incorrect by making this change on main

- const config = configFromFs(['_layout.tsx', 'bar.tsx', 'index.tsx']);
+ const config = configFromFs(['./_layout.tsx', './bar.tsx', './index.tsx']);

expect(getStateFromPath<object>(path, config)).toEqual({
-  routes: [{ name: '', state: { routes: [{ name: 'bar', path: '/bar' }] } }],
+  routes: [{ name: 'bar', path: '/bar' }],
});

I've updated the getPathsFromState to use a new helper getMockState that utilises the getMockContext function.

A future advantage of this refactor is the ability to test the configs being generated with unstable_settings

const config = getMockConfig({
  page: {
    unstable_settings: {},
    default: () => null
  }
})

Test Plan

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[EvanBacon]

It would be nice if this function also accepted an array of strings for brevity.

[marklawlor]

Done & updated the documentation