Suggest `--ignored` when libtest ignores a test

[jyn514]

Code

; cat src/lib.rs
#[test]
#[ignore = "takes too long 😩"]
fn i_am_slow() {
    std::thread::sleep(std::time::Duration::from_secs(5));
    println!("it works!");
}
; cargo test --lib --quiet

Current output

running 1 test
i
test result: ok. 0 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.00s

test: i_am_slow, ignore_message: takes too long 😩

Desired output

running 1 test
i
test result: ok. 0 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.00s

test: i_am_slow, ignore_message: takes too long 😩

note: to run ignored tests, use `--ignored`

Rationale and extra context

See rust-lang/cargo#12494; currently this will be confusing for people using cargo so I'd like to fix that issue first.

Other cases

If no tests are ignored, this help message shouldn't be printed.

Anything else?

No response

[RickleAndMortimer]

@rustbot claim