error reusing channel for running parallel client calling the same server in `tokio::test`

## Bug Report

### Version

tonic = "0.12.3"
tonic-health = "0.12.3"

### Platform

Darwin MacBook-Pro-64.local 21.6.0 Darwin Kernel Version 21.6.0: Wed Aug 10 14:28:23 PDT 2022; root:xnu-8020.141.5~2/RELEASE_ARM64_T6000 arm64

### Description

I'm trying to run integration tests that will query the health check of a server already running.

lib.rs:
```rust
pub struct ThreadSafeSigner(OnceCell);

#[fixture]
#[once]
pub fn signer() -> ThreadSafeSigner {
ThreadSafeSigner(OnceCell::default())
}

impl ThreadSafeSigner {
async fn get_signer_channel(&self) -> Result {
let signer_port = std::env::var("SOCKET_PORT")?;

let channel = self
.0
.get_or_try_init::(|| async {
let address = format!("http://localhost:{}", signer_port);

let timeout = Instant::now() + Duration::from_secs(3);
let channel = loop {
if let Ok(c) = tonic::transport::Channel::builder(address.parse()?)
.connect()
.await
{
break c;
}
if Instant::now() > timeout {
return Err(anyhow!("timeout waiting for signer"));
}
};

Ok(channel)
})
.await?;

Ok(channel.clone())
}

pub async fn init_health_client(&self) -> Result> {
let channel = self.get_signer_channel().await?;
let client = tonic_health::pb::health_client::HealthClient::new(channel);

Ok(client)
}
}
```

test.rs
```rust
#[rstest]
#[tokio::test]
async fn is_healthy0(signer: &ThreadSafeSigner) -> Result<()> {
let mut client = signer.init_health_client().await?;
let res = client
.check(HealthCheckRequest {
service: "signer.Signer".to_string(),
})
.await?;
let serving_status = ServingStatus::try_from(res.into_inner().status)?;

if serving_status == ServingStatus::Serving {
Ok(())
} else {
Err(anyhow!(
"invalid status, expected SERVING, got {}",
serving_status.as_str_name()
))
}
}

...

```

I do this because the doc recommends cloning the channel and reuse it.

I get the following error:
```shell

running 0 tests

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

Running declare_keyset.rs (target/debug/deps/declare_keyset-aa3fe48cf645a906)

running 10 tests
test is_healthy2 ... ok
test is_healthy1 ... ok
test is_healthy6 ... ok
test is_healthy0 ... ok
test is_healthyc5 ... ok
test is_healthy7 ... ok
test is_healthy8 ... ok
Error: status: Unknown, message: "transport error", details: [], metadata: MetadataMap { headers: {} }

Caused by:
0: transport error
1: dispatch task is gone
2: runtime dropped the dispatch task
Error: status: Unknown, message: "transport error"test is_healthy4 ... FAILED
, details: [], metadata: MetadataMap { headers: {} }

Caused by:
0: transport error
1: dispatch task is gone
2: runtime dropped the dispatch task
test is_healthy6n ... ok
test is_healthy3 ... FAILED

failures:

failures:
is_healthy3
is_healthy4

test result: FAILED. 8 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

error: test failed, to rerun pass `-p signer-tests --test declare_keyset`
```

Sometime it goes through, sometime 1 or 2 fail, sometimes all.

What am I doing incorrectly?