Schematics: Unit testing a rule that uses mergeWith/apply fails due to missing context

## Addendum

In creating the test case I've realised that I'm not passing a `partialContext` to `callRule`, so it is definitely missing context, however, it is not at all clear _what_ context is necessary for `mergeWith` to succeed and the error does not indicate that context is actually missing.

Happy for this to be marked as a question instead. Guidance on _how_ to provide the context and _what_ context to provide would be amazingly useful.

# 🐞 Bug report

### Description

Using `callRule` to unit test rules within a schematic fails when it comes to rules which return `mergeWith`

## 🔬 Minimal Reproduction

See: https://github.com/jdpearce/schematics-apply-problem

Given a rule which applies template files to the tree, this unit test will fail with `TypeError: Cannot read property 'path' of undefined`:

```ts
describe('applyTemplateFiles Rule', () => {
let tree: UnitTestTree;

beforeEach(async () => {
tree = new UnitTestTree(Tree.empty());
});

it('should apply the files to the tree', async () => {
const schema = {};

tree = (await callRule(applyTemplateFiles(schema), tree)) as UnitTestTree;

expect(tree.files).toEqual(jasmine.arrayContaining(['/libs/destination/test.txt']));
});
});
```

## 🔥 Exception or Error

```bash
Failures:
1) applyTemplateFiles Rule should apply the files to the tree
Message:
TypeError: Cannot read property 'path' of undefined
Stack:
at
at ./apply-problem/node_modules/@angular-devkit/schematics/tools/file-system-engine-host-base.js:216:96
at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/url.js:13:73
at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)
at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:45:60
at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)
at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:53:23
at MergeMapSubscriber.project (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:74:24)
at MergeMapSubscriber._tryNext (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:69:27)
at MergeMapSubscriber._next (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:59:18)
at MergeMapSubscriber.Subscriber.next (./tmp/apply-problem/node_modules/rxjs/internal/Subscriber.js:66:18)

2 specs, 1 failure
```

## 🌍 Your Environment

"@angular-devkit/core": "^9.0.6",
"@angular-devkit/schematics": "^9.0.6",
"@types/jasmine": "^3.3.9",
"@types/node": "^8.0.31",
"jasmine": "^3.3.1",
"typescript": "~3.5.3"

Start with the linked minimal reproduction and the unit test using callRule, mergeWith, and apply. Trace the reported failure through file-system-engine-host-base.js and the schematics url.js entry point, paying attention to the omitted partialContext. Done should establish the required context and either make the test pass or clearly report the missing context.

Written by the indexing model from the issue text.