`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

Commencez par karma.conf.js et le point d’entrée de ng test, en utilisant la configuration urlRoot indiquée pour reproduire l’échec. Suivez la façon dont le fichier de contexte personnalisé résout context.js lorsque urlRoot est défini sur newBase, et comparez-la au chemin attendu /karma/context.js. Le travail est terminé lorsque l’exécution du test demande le script de contexte sous le urlRoot configuré sans 404.

Rédigé par le modèle d'indexation à partir du texte de l'issue.