Add test for impl of available_parallelism()

src/test/ui/thread-available_parallelism.rs

@@ -0,0 +1,12 @@
+// run-pass
+// this is an ignore list on purpose so that new platforms don't miss it:
+//ignore-vxworks
+//ignore-redox
+//ignore-l4re
+
+// check that std::thread::available_parallelism returns a valid value
+
+fn main() {
+    std::thread::available_parallelism().expect(
+        "should return a value on all platforms that are not ignored");
+}