Fix for for setting test-threads to 0

Running test with cargo test -- --test-threads=0 causes cargo to
hang as 0 is a valid usize. Adding zero threads as a special case
to the error handling.

src/libtest/lib.rs

@@ -439,6 +439,8 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
     let test_threads = match matches.opt_str("test-threads") {
         Some(n_str) =>
             match n_str.parse::<usize>() {
+                Ok(0) => 
+                    return Some(Err(format!("argument for --test-threads must not be 0"))),
                 Ok(n) => Some(n),
                 Err(e) =>
                     return Some(Err(format!("argument for --test-threads must be a number > 0 \