ng test: Collect total code coverage in multi project repository

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Area
```
- [x] devkit
- [ ] schematics
```

### Versions

```
Angular CLI: 6.0.8
Node: 8.9.1
OS: win32 x64
Angular: 5.2.11
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

Package Version
-----------------------------------------------------------
@angular-devkit/architect 0.6.8
@angular-devkit/build-angular 0.6.8
@angular-devkit/build-optimizer 0.6.8
@angular-devkit/core 0.6.8
@angular-devkit/schematics 0.6.8
@angular/cdk 5.2.5
@angular/cli 6.0.8
@ngtools/json-schema 1.1.0
@ngtools/webpack 6.0.8
@schematics/angular 0.6.1
@schematics/update 0.6.8
ng-packagr 3.0.2
rxjs 6.2.1
typescript 2.6.2
webpack 4.8.3
```

### Repro steps

Multi project workspace, run `ng test` to test all projects.

### The log given by the failure

Produced coverage report in `coverage/lcov.info` only contains results for the last run code coverage.

HTML reports are stored per project in `coverage/`.

### Desired functionality

Write indivudual lcovs per project, e.g, `coverage//lcov.info`, allowing users to get a total code coverage for the whole project.

Alternative would be functionality similar to [istanbul-combine](https://github.com/jamestalmage/istanbul-combine). Write `coverage.json` for each project and write a combined coverage report and end of test runs.

### Mention any other details that might be useful

Should eht changes be made in the builder or in the schematic?

https://github.com/angular/angular-cli/blob/master/packages/schematics/angular/library/files/__projectRoot__/karma.conf.js#L19

https://github.com/angular/angular-cli/blob/master/packages/schematics/angular/application/files/root/karma.conf.js#L19

Beginne damit, den Multi-Projekt-Workspace-Fall mit `ng test` zu reproduzieren, und untersuche dann die referenzierten `karma.conf.js`-Dateien unter den Application- und Library-Schematics. Verfolge, ob die Handhabung der Coverage in den Builder oder das Schematic gehört; abgeschlossen ist die Aufgabe, wenn jedes Projekt eigene lcov-Daten hat und ein kombinierter Report die Gesamt-Coverage darstellen kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.