`urlRoot` in `karma.conf.js` can't be set to anything other than the default `/`

# 🐞 Bug report

### Command (mark with an `x`)

- [ ] new
- [ ] build
- [ ] serve
- [x] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] extract-i18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Is this a regression?

No.

### Description

Can't set `urlRoot` in `karma.conf.js` to anything other than "/" because:

> Angular CLI uses custom context file, which leads to this bug. It adds ``, which forces context.html to request http://localhost:9876/context.js instead of http://localhost:9876/karma/context.js.

which is described in detail in this issue: https://github.com/karma-runner/karma/issues/3357#issuecomment-634630974.

## 🔬 Minimal Reproduction

Run `ng test` with a `karma.conf.js` that contains a `urlRoot` set to anything but `/`:

```js
{
...,
urlRoot: "newBase"
}

```

## 🔥 Exception or Error

The error that is logged is the following:

```
09 11 2021 23:25:29.766:WARN [web-server]: 404: /_karma_webpack_/context.js
```

## 🌍 Your Environment

Angular CLI: 11.2.0

Node: 14.18.0

OS: darwin x64


Angular: 11.2.14

... common, compiler, compiler-cli, core, forms, localize

... platform-browser, platform-browser-dynamic, router, upgrade

Ivy Workspace: Yes

Package                         Version

---------------------------------------------------------

@angular-devkit/architect       0.1102.0

@angular-devkit/build-angular   0.1102.0

@angular-devkit/core            11.2.14

@angular-devkit/schematics      11.2.14

@angular/animations             12.2.8

@angular/cdk                    11.2.13

@angular/cli                    11.2.0

@schematics/angular             11.2.14

@schematics/update              0.1102.0

rxjs                            6.6.7

typescript                      4.1.6

karma.conf.js と ng test のエントリポイントから始め、示されている urlRoot の設定を使って失敗を再現します。urlRoot が newBase に設定されているときに、カスタムコンテキストファイルが context.js をどのように解決するかを追跡し、期待される /karma/context.js パスと比較します。テスト実行が、設定された urlRoot 配下のコンテキストスクリプトを 404 なしでリクエストすれば完了です。

索引モデルが issue の本文から書いたものです。