`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

Comienza con karma.conf.js y el punto de entrada de ng test, usando la configuración de urlRoot mostrada para reproducir el fallo. Sigue cómo el archivo de contexto personalizado resuelve context.js cuando urlRoot está establecido en newBase y compáralo con la ruta esperada /karma/context.js. Se considera terminado cuando la ejecución de la prueba solicita el script de contexto bajo el urlRoot configurado sin un 404.

Escrito por el modelo de indexación a partir del texto del issue.