Test cleanup: spawn command

[weedySeaDragon]

• Added tests for spawn and spawnSync
• Changed existing tests for spawnCommand and spawnCommandSync: they call spawn and spawnSync
• changed the name of the generator used for testing from "spawn" to "testGenerator." The word "spawn" was used too much (ex: spawn.spawn(..) is not very clear)

Part of test cleanup: #1481

[mshima]

Thanks @weedySeaDragon