threads2: Assert jobs_done == 10 to reject non-shared solutions (#2200)

[GolakiyaVishal]

Fixes #2200

Problem

threads2 is configured with test = false, same as other exercises. So Rustlings only checks that the exercise compile and runs. There for user able to complete it many ways.

Changes

• Now computes jobs_done as a variable instead of inclining it in println!.
• Added assert_eq! at the end of main so it actually check the jobs_done. So if the shared state not change then it will panic and exercise fails.
• Updated solutions/20_threads/threads2.rs to match the new structure.

Testing

• The official Arc<Mutex<JobStatus>> solution passes.
• The Arc::make_mut approach from the issue now fails with the assertion message.
• cargo run -- dev check passes.