test new available_concurrency function

Author: RalfJung

rust-version

@@ -1 +1 @@
-043eca7f0b34d12e61c44206beca740628647080
+c38ddb8040edce1b05bc09a0e8439472e9f67623

tests/run-pass/available-concurrency.rs

@@ -0,0 +1,5 @@
+#![feature(available_concurrency)]
+
+fn main() {
+    assert_eq!(std::thread::available_concurrency().unwrap().get(), 1);
+}