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

先使用 `ng test` 重現多專案 workspace 的情況，然後檢查 application 和 library schematics 下所參照的 `karma.conf.js` 檔案。追蹤 coverage 的處理應歸屬於 builder 還是 schematic；完成的條件是每個專案都有自己的 lcov 資料，且合併報告能夠呈現整體 coverage。

由索引模型根據 Issue 內容生成。