HW_AVAILCPU is unavailable under openbsd

define `num_cpus()` function for openbsd that use `HW_NCPU` for grabbing
the current number of cpus that could be used.

src/libtest/lib.rs

@@ -919,7 +919,6 @@ fn get_concurrency() -> usize {
     #[cfg(any(target_os = "freebsd",
               target_os = "dragonfly",
               target_os = "bitrig",
-              target_os = "openbsd",
               target_os = "netbsd"))]
     fn num_cpus() -> usize {
         let mut cpus: libc::c_uint = 0;
@@ -946,6 +945,24 @@ fn get_concurrency() -> usize {
         }
         cpus as usize
     }
+
+    #[cfg(target_os = "openbsd")]
+    fn num_cpus() -> usize {
+        let mut cpus: libc::c_uint = 0;
+        let mut cpus_size = std::mem::size_of_val(&cpus);
+        let mut mib = [libc::CTL_HW, libc::HW_NCPU, 0, 0];
+
+        unsafe {
+            libc::sysctl(mib.as_mut_ptr(), 2,
+                         &mut cpus as *mut _ as *mut _,
+                         &mut cpus_size as *mut _ as *mut _,
+                         0 as *mut _, 0);
+        }
+        if cpus < 1 {
+            cpus = 1;
+        }
+        cpus as usize
+    }
 }
 
 pub fn filter_tests(opts: &TestOpts, tests: Vec<TestDescAndFn>) -> Vec<TestDescAndFn> {