Add test for fillCacheWithNewSubTreeData (#45298)

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Follow up to moving router-reducer. This adds a unit test for injecting
the subTreeData.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/src/client/components/router-reducer/fill-cache-with-new-subtree-data.test.tsx

@@ -0,0 +1,152 @@
+import React from 'react'
+import { fillCacheWithNewSubTreeData } from './fill-cache-with-new-subtree-data'
+import { CacheStates, CacheNode } from '../../../shared/lib/app-router-context'
+import { FlightData } from '../../../server/app-render'
+
+const getFlightData = (): FlightData => {
+  return [
+    [
+      'children',
+      'linking',
+      'children',
+      'about',
+      [
+        'about',
+        {
+          children: ['', {}],
+        },
+      ],
+      <h1>SubTreeData Injected!</h1>,
+      <>
+        <title>Head Injected!</title>
+      </>,
+    ],
+  ]
+}
+
+describe('fillCacheWithNewSubtreeData', () => {
+  it('should apply subTreeData and head property', () => {
+    const cache: CacheNode = {
+      status: CacheStates.LAZY_INITIALIZED,
+      data: null,
+      subTreeData: null,
+      parallelRoutes: new Map(),
+    }
+    const existingCache: CacheNode = {
+      data: null,
+      status: CacheStates.READY,
+      subTreeData: <>Root layout</>,
+      parallelRoutes: new Map([
+        [
+          'children',
+          new Map([
+            [
+              'linking',
+              {
+                data: null,
+                status: CacheStates.READY,
+                subTreeData: <>Linking</>,
+                parallelRoutes: new Map([
+                  [
+                    'children',
+                    new Map([
+                      [
+                        '',
+                        {
+                          data: null,
+                          status: CacheStates.READY,
+                          subTreeData: <>Page</>,
+                          parallelRoutes: new Map(),
+                        },
+                      ],
+                    ]),
+                  ],
+                ]),
+              },
+            ],
+          ]),
+        ],
+      ]),
+    }
+
+    const flightData = getFlightData()
+
+    if (typeof flightData === 'string') {
+      throw new Error('invalid flight data')
+    }
+
+    // Mirrors the way router-reducer values are passed in.
+    const flightDataPath = flightData[0]
+
+    fillCacheWithNewSubTreeData(cache, existingCache, flightDataPath)
+
+    const expectedCache: CacheNode = {
+      data: null,
+      status: CacheStates.LAZY_INITIALIZED,
+      subTreeData: null,
+      parallelRoutes: new Map([
+        [
+          'children',
+          new Map([
+            [
+              'linking',
+              {
+                data: null,
+                status: CacheStates.READY,
+                subTreeData: <>Linking</>,
+                parallelRoutes: new Map([
+                  [
+                    'children',
+                    new Map([
+                      // TODO-APP: this segment should be preserved when creating the new cache
+                      [
+                        '',
+                        {
+                          data: null,
+                          status: CacheStates.READY,
+                          subTreeData: <>Page</>,
+                          parallelRoutes: new Map(),
+                        },
+                      ],
+                      [
+                        'about',
+                        {
+                          data: null,
+                          parallelRoutes: new Map([
+                            [
+                              'children',
+                              new Map([
+                                [
+                                  '',
+                                  {
+                                    data: null,
+                                    status: CacheStates.LAZY_INITIALIZED,
+                                    subTreeData: null,
+                                    parallelRoutes: new Map(),
+                                    head: (
+                                      <>
+                                        <title>Head Injected!</title>
+                                      </>
+                                    ),
+                                  },
+                                ],
+                              ]),
+                            ],
+                          ]),
+                          subTreeData: <h1>SubTreeData Injected!</h1>,
+                          status: CacheStates.READY,
+                        },
+                      ],
+                    ]),
+                  ],
+                ]),
+              },
+            ],
+          ]),
+        ],
+      ]),
+    }
+
+    expect(cache).toMatchObject(expectedCache)
+  })
+})

packages/next/src/client/components/router-reducer/fill-lazy-items-till-leaf-with-head.test.tsx

@@ -88,8 +88,6 @@ describe('fillLazyItemsTillLeafWithHead', () => {
     const flightDataPath = flightData[0]
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
     const [treePatch, _subTreeData, head] = flightDataPath.slice(-3)
-    // const flightSegmentPath = flightDataPath.slice(0, -4)
-
     fillLazyItemsTillLeafWithHead(cache, existingCache, treePatch, head)
 
     const expectedCache: CacheNode = {