chore: Electron code coverage setup

[bennycode]

Let me describe the code coverage workflow which makes it possible to get the coverage metrics for code that runs inside Electron's main process:

1. TypeScript source code gets compiled with tsc
2. Compiled code gets instrumented with istanbul
3. Instrumented code will be tested using electron-mocha within Electron's main process
4. A script bound to mocha's global "after" hook will write the coverage info into a coverage.json file
5. remap-istanbul will create a nice HTML report based on coverage.json and remap the compiled code to it's TypeScript sources within the report
6. Bonus: Code coverage thresholds will be checked using istanbul check-coverage

Although the workflow sounds straight forward, there are some caveats:

1. Within the tests we need to import the instrumented code and not the compiled code → electron-mocha and istanbul/isparta jprichardson/electron-mocha#19 (comment)
2. To get proper coverage reports of the full code base, we need to require every code that is being used which is done in BeforeTest.js using mocha's before hook → How to include all source files for code coverage reporting? jprichardson/electron-mocha#135

Special adjustments to make it fit with our codebase:

1. Tests will be run from the ./electron subdirectory because we need the node modules from there plus the references to ./electron/locale
2. To prevent tests in Electron's main process from crashing when loading code that needs to be run in Electron's renderer process, I moved all renderer-related code into ./electron/src/renderer and excluded these fragments in BeforeTest.js
3. I moved the configuration for electron-mocha into mocha.opts, which is the default file to store configurations for mocha

[atomrc]

maybe this script can be renamed just test. It is not directly linked to coverage, am I wrong?

[bennycode]

Ok, has been renamed: 6f4f2e6