Add --main-process flag to run specs in the main process

…so that we can write unit tests for code in the main process. Below a
list of conventions we'll be adopting:

1. Use mocha as the test framework.
2. Use chai as the assertion library.
3. Name specs `foo-bar.spec.js`, to prevent the renderer process from
running specs written fro the main process e.g. when calling
`atom --test spec`.
4. Write specs in ES6.

Although somewhat inconsistent with the conventions we use for renderer
process specs, this will hopefully be a first step towards migrating our
entire Jasmine 1.3 test suite to a more modern environment.

src/browser/main.coffee

@@ -9,6 +9,7 @@ fs = require 'fs-plus'
 path = require 'path'
 temp = require 'temp'
 yargs = require 'yargs'
+previousConsoleLog = console.log
 console.log = require 'nslog'
 
 start = ->
@@ -16,7 +17,13 @@ start = ->
   args.env = process.env
   setupAtomHome(args)
   setupCompileCache()
-  return if handleStartupEventWithSquirrel()
+  if handleStartupEventWithSquirrel()
+    return
+  else if args.test and args.mainProcess
+    console.log = previousConsoleLog
+    resourcePath = if fs.existsSync(args.devResourcePath) then args.devResourcePath else require.resolve('../..')
+    require(path.join(args.devResourcePath, 'spec/browser/mocha-test-runner'))(args.pathsToOpen)
+    return
 
   # NB: This prevents Win10 from showing dupe items in the taskbar
   app.setAppUserModelId('com.squirrel.atom.atom')
@@ -130,6 +137,7 @@ parseCommandLine = ->
   options.boolean('safe').describe('safe', 'Do not load packages from ~/.atom/packages or ~/.atom/dev/packages.')
   options.boolean('portable').describe('portable', 'Set portable mode. Copies the ~/.atom folder to be a sibling of the installed Atom location if a .atom folder is not already there.')
   options.alias('t', 'test').boolean('t').describe('t', 'Run the specified specs and exit with error code on failures.')
+  options.alias('m', 'main-process').boolean('m').describe('m', 'Run the specified specs in the main process.')
   options.string('timeout').describe('timeout', 'When in test mode, waits until the specified time (in minutes) and kills the process (exit code: 130).')
   options.alias('v', 'version').boolean('v').describe('v', 'Print the version information.')
   options.alias('w', 'wait').boolean('w').describe('w', 'Wait for window to be closed before returning.')
@@ -154,6 +162,7 @@ parseCommandLine = ->
   safeMode = args['safe']
   pathsToOpen = args._
   test = args['test']
+  mainProcess = args['main-process']
   timeout = args['timeout']
   newWindow = args['new-window']
   pidToKillWhenClosed = args['pid'] if args['wait']
@@ -186,6 +195,6 @@ parseCommandLine = ->
   {resourcePath, devResourcePath, pathsToOpen, urlsToOpen, executedFrom, test,
    version, pidToKillWhenClosed, devMode, safeMode, newWindow,
    logFile, socketPath, userDataDir, profileStartup, timeout, setPortable,
-   clearWindowState, addToLastWindow}
+   clearWindowState, addToLastWindow, mainProcess}
 
 start()