Add test for available_parallelism()

This is a redo of (this PR)[#104095].

library/std/tests/thread.rs

@@ -37,3 +37,11 @@ fn thread_local_containing_const_statements() {
     assert_eq!(CELL.get(), 1);
     assert_eq!(REFCELL.take(), 1);
 }
+
+#[test]
+// Include an ignore list on purpose, so that new platforms don't miss it
+#[cfg(not(any(target_os = "vxworks", target_os = "redox", target_os = "l4re")))]
+fn available_parallelism() {
+    // check that std::thread::available_parallelism() returns a valid value
+    assert!(thread::available_parallelism().is_ok());
+}