Test fixes from rollup

Closes #14210 (Make Vec.truncate() resilient against failure in Drop)
Closes #14206 (Register new snapshots)
Closes #14205 (use sched_yield on linux and freebsd)
Closes #14204 (Add a crate for missing stubs from libcore)
Closes #14203 (shootout-mandelbrot: Either 10-20% or 80-100% improvement.)
Closes #14201 (Render not_found with an absolute path to the rust stylesheet)
Closes #14198 (update valgrind headers)
Closes #14174 (Optimize common path of Once::doit)
Closes #14162 (Print 'rustc' and 'rustdoc' as the command name for --version)
Closes #14145 (Better strict version hash (SVH) computation)
Closes #14133 (define Eq,TotalEq,Ord,TotalOrd for &mut T)
Closes #14121 (Make some NullablePointer enums FFI-compatible with the base pointer type.)