Fixes:

* give the new feature its own feature tag
 * correct a lifetime problem in the test
 * use .output() instead of .spawn() in the test so that output is
   actually collected
 * correct the same error in the test whose skeleton I cribbed

src/libstd/process.rs

@@ -399,7 +399,7 @@ impl Command {
     ///         .spawn()
     ///         .expect("printenv failed to start");
     /// ```
-    #[stable(feature = "process", since = "1.16.0")]
+    #[stable(feature = "command_envs", since = "1.16.0")]
     pub fn envs<K, V>(&mut self, vars: &[(K, V)]) -> &mut Command
         where K: AsRef<OsStr>, V: AsRef<OsStr>
     {