Fix bug where the outer num_ignored variable was ignored.

[ltratt]

The basic problem is that Rust allows move closure to capture variables if they
have a Copyable type, leading to the "outer" and "inner" variables to diverge.
For example, this simple program:

  fn main() {
    let mut x = 0;
    let mut y = move |z: u32| {
      x += z;
      println!("inner x {}", x);
    };

    println!("outer x {}", x);
    y(2);
    println!("outer x {}", x);
  }

outputs:

  outer x 0
  inner x 2
  outer x 0

Note that the above program gives no warnings (or errors) in rustc or clippy.

In this case, the num_ignored variable had started life in single-threaded
code and later been moved to multi-threaded code. It compiled, even though the
outer variable could never be mutated. In other words, lang_tester always said
that 0 tests were ignored -- even if some were ignored!

This commit fixes this problem using an AtomicUsize so that ignored tests now
count as can be seen here:

  $ cargo run --example rust_lang_tester
      Finished dev [unoptimized + debuginfo] target(s) in 0.12s
       Running `target/debug/examples/rust_lang_tester`

  running 7 tests
  test lang_tests::no_main ... ok
  test lang_tests::ignore ... ignored
  test lang_tests::unknown_var ... ok
  test lang_tests::exit_code ... ok
  test lang_tests::unused_var ... ok
  test lang_tests::sig_caught ... ok
  test lang_tests::custom_cla ... ok

  test result: ok. 7 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out

[jacob-hughes]

LGTM