Align with rustc/compiletest

[laumann]

The current codebase is quite out-of-sync with the version in rustc.

Align the two implementations again. Keep in mind:

• The stable feature is an add-on that might cause issues
• The tmp feature

Most of the code that deviates from/adds to rustc's compiletest is found in common.rs, but some of it has snuck into runtest.rs (and maybe other places).