Auto-testing of GitHub pull requests

[Joseph Rushton Wakeling]

Hello all,

GitHub very nicely ensures that all pull requests to dmd, druntime and phobos 
are sent to the auto-tester and that success/failure reports are sent back.

But ... how is this done?  Is the auto-tester entirely custom written or does it 
use some standard tools?  And how is it hooked into GitHub?

Thanks & best wishes,

     -- Joe