Use user-data-dir with temp directory when running core tests

This is to ensure successful database connection. Since core specs and 
package specs are run at the same time and both open an indexedDB 
connection, there were occasional core spec failures due to failed
database connection.

build/tasks/spec-task.coffee

@@ -1,5 +1,6 @@
 fs = require 'fs'
 path = require 'path'
+temp = require('temp').track()
 
 _ = require 'underscore-plus'
 async = require 'async'
@@ -94,7 +95,7 @@ module.exports = (grunt) ->
     if process.platform in ['darwin', 'linux']
       options =
         cmd: appPath
-        args: ['--test', "--resource-path=#{resourcePath}", coreSpecsPath]
+        args: ['--test', "--resource-path=#{resourcePath}", coreSpecsPath, "--user-data-dir=#{temp.mkdirSync('atom-user-data-dir')}"]
         opts:
           env: _.extend({}, process.env,
             ATOM_INTEGRATION_TESTS_ENABLED: true