std: Move a process test out of libstd

This commit moves a test out of libstd which is causing deadlocks on
musl on CI. Looks like the recent update in musl versions brings in some
internal updates to musl which makes `setgid` and `setuid` invalid to
call after a `fork` in a multithreaded program. The issue seen here is
that the child thread was attempting to grab a lock held by a
nonexistent thread, meaning that the child process simply deadlocked
causing the whole test to deadlock.

This commit moves the test to its own file with no threads which should
work.

src/libstd/process.rs

@@ -1765,33 +1765,6 @@ mod tests {
         assert_eq!(out, "foobar\n");
     }
 
-
-    #[test]
-    #[cfg_attr(target_os = "android", ignore)]
-    #[cfg(unix)]
-    fn uid_works() {
-        use crate::os::unix::prelude::*;
-
-        let mut p = Command::new("/bin/sh")
-                            .arg("-c").arg("true")
-                            .uid(unsafe { libc::getuid() })
-                            .gid(unsafe { libc::getgid() })
-                            .spawn().unwrap();
-        assert!(p.wait().unwrap().success());
-    }
-
-    #[test]
-    #[cfg_attr(target_os = "android", ignore)]
-    #[cfg(unix)]
-    fn uid_to_root_fails() {
-        use crate::os::unix::prelude::*;
-
-        // if we're already root, this isn't a valid test. Most of the bots run
-        // as non-root though (android is an exception).
-        if unsafe { libc::getuid() == 0 } { return }
-        assert!(Command::new("/bin/ls").uid(0).gid(0).spawn().is_err());
-    }
-
     #[test]
     #[cfg_attr(target_os = "android", ignore)]
     fn test_process_status() {

src/test/run-pass/command-uid-gid.rs

@@ -0,0 +1,26 @@
+#![feature(rustc_private)]
+
+fn main() {
+    #[cfg(unix)]
+    run()
+}
+
+#[cfg(unix)]
+fn run() {
+    extern crate libc;
+    use std::process::Command;
+    use std::os::unix::prelude::*;
+
+    let mut p = Command::new("/bin/sh")
+        .arg("-c").arg("true")
+        .uid(unsafe { libc::getuid() })
+        .gid(unsafe { libc::getgid() })
+        .spawn().unwrap();
+    assert!(p.wait().unwrap().success());
+
+    // if we're already root, this isn't a valid test. Most of the bots run
+    // as non-root though (android is an exception).
+    if unsafe { libc::getuid() != 0 } {
+        assert!(Command::new("/bin/ls").uid(0).gid(0).spawn().is_err());
+    }
+}