Test fixes from rollup

Closes #14231 (mk: Don't run benchmarks with `make check`)
Closes #14215 (std: Modify TempDir to not fail on drop. Closes #12628)
Closes #14211 (rustdoc: functions in ffi blocks are unsafe)
Closes #14210 (Make Vec.truncate() resilient against failure in Drop)
Closes #14208 (Make `from_bits` in `bitflags!` safe; add `from_bits_truncate`)
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 #14202 (Add flow-graph visualization (via graphviz) to rustc)
Closes #14201 (Render not_found with an absolute path to the rust stylesheet)
Closes #14200 (std cleanup)
Closes #14189 (Implement cell::clone_ref)

Author: alexcrichton

src/libgreen/sched.rs

@@ -1137,7 +1137,6 @@ mod test {
     fn test_schedule_home_states() {
         use sleeper_list::SleeperList;
         use super::{Shutdown, Scheduler, SchedHandle};
-        use std::unstable::Thread;
         use std::rt::thread::Thread;
         use std::sync::deque::BufferPool;
 

src/librustuv/lib.rs

@@ -472,7 +472,7 @@ fn local_loop() -> &'static mut uvio::UvIoFactory {
 #[cfg(test)]
 mod test {
     use std::mem::transmute;
-    use std::rt::Thread;
+    use std::rt::thread::Thread;
 
     use super::{slice_to_uv_buf, Loop};
 

src/librustuv/uvio.rs

@@ -27,7 +27,7 @@ use std::rt::rtio;
 use std::rt::rtio::{ProcessConfig, IoFactory, EventLoop};
 use ai = std::io::net::addrinfo;
 
-#[cfg(test)] use std::rt::Thread;
+#[cfg(test)] use std::rt::thread::Thread;
 
 use super::{uv_error_to_io_error, Loop};