Run postgres directly and not via pg_ctl in Linux

[byrnedo]

Will ensure the process dies. Windows behaviour unchanged since it's more involved.

[fergusstrange]

I think the only way we're getting this merged is having consistent behaviour for all platforms. The extra code involved means we're moving away from the goal of this library, which is to deliver a consistent testing performance for all platforms. I suspect we could solve the entire problem with some smart test wrappers that caught any panics and ensured a PID was removed if it was left hanging around on all systems?