Auto merge of #24177 - alexcrichton:rustdoc, r=aturon

This commit series starts out with more official test harness support for rustdoc tests, and then each commit afterwards adds a test (where appropriate). Each commit should also test and finish independently of all others (they're all pretty separable).

I've uploaded a [copy of the documentation](http://people.mozilla.org/~acrichton/doc/std/) generated after all these commits were applied, and a double check on issues being closed would be greatly appreciated! I'll also browse the docs a bit and make sure nothing regressed too horribly.

src/libstd/process.rs

@@ -38,8 +38,6 @@ use thread;
 /// # Examples
 ///
 /// ```should_panic
-/// # #![feature(process)]
-///
 /// use std::process::Command;
 ///
 /// let output = Command::new("/bin/cat").arg("file.txt").output().unwrap_or_else(|e| {
@@ -267,10 +265,8 @@ impl Command {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(process)]
     /// use std::process::Command;
-    ///
-    /// let output = Command::new("cat").arg("foot.txt").output().unwrap_or_else(|e| {
+    /// let output = Command::new("cat").arg("foo.txt").output().unwrap_or_else(|e| {
     ///     panic!("failed to execute process: {}", e)
     /// });
     ///
@@ -291,7 +287,6 @@ impl Command {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(process)]
     /// use std::process::Command;
     ///
     /// let status = Command::new("ls").status().unwrap_or_else(|e| {