allow user disable electron's sandbox

[bruceauyeung]

why user need to disable electron's sandbox :

1. users want to run jest/jest-electron as root. without --no-sandbox option for electron , test cases will fail silently because jest-electron failed to spawn an electron process
2. Users of cypress have reported that the new Electron v5 seems to require --no-sandbox flag even when running as a regular non-root user. see Always use --no-sandbox with Electron on non-Windows system cypress-io/cypress#5458 (comment)

security impact:

malicious javascript codes running within electron's renderer process are not sandboxed anymore and have the ability to harm the system. but here in jest test cases are trusted codes, so i think security impact is trivial.

who disables electron's sandbox on non-windows platform by default:

1, vscode. see microsoft/vscode#81096
2, cypress. see cypress-io/cypress#5458

fixes #19

[hustcc]

@bruceauyeung do the latest version of electron supports --no-sandbox ? I will upgrade electron to latest version.