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

Bắt đầu bằng cách tái hiện trường hợp workspace nhiều project với `ng test`, sau đó kiểm tra các tệp `karma.conf.js` được tham chiếu bên dưới các application và library schematics. Theo dõi xem việc xử lý coverage thuộc về builder hay schematic; hoàn thành khi mỗi project có dữ liệu lcov riêng và một báo cáo kết hợp có thể biểu diễn coverage tổng thể.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.