Revert "std::rt: Use a constant 4 threads for multithreaded sched tests"

This workaround was less than ideal. A better solution is to raise the
fd limit.

This reverts commit 49b72bdd77916e27aaf95909516702c1450f11ac.

src/libstd/rt/test.rs

@@ -70,6 +70,7 @@ pub fn run_in_mt_newsched_task(f: ~fn()) {
     use os;
     use from_str::FromStr;
     use rt::sched::Shutdown;
+    use rt::util;
 
     let f = Cell::new(f);
 
@@ -77,10 +78,10 @@ pub fn run_in_mt_newsched_task(f: ~fn()) {
         let nthreads = match os::getenv("RUST_RT_TEST_THREADS") {
             Some(nstr) => FromStr::from_str(nstr).get(),
             None => {
-                // A reasonable number of threads for testing
-                // multithreading. NB: It's easy to exhaust OS X's
-                // low maximum fd limit by setting this too high (#7772)
-                4
+                // Using more threads than cores in test code
+                // to force the OS to preempt them frequently.
+                // Assuming that this help stress test concurrent types.
+                util::num_cpus() * 2
             }
         };