feat: allow getting the Assert from an AssertError

[jbtrystram]

I'm in the process of writing integration tests with this awesome crate :)

My tool is basically a CRUD cli for a distant API, and while writing tests I ran into this problem while testing deletions.
In some cases, i need to check the output of the Assert even if it succeeded when I expected a failure ( but that would work the other way around.

An example : to verify a deletion process you'd do :delete, then read and expect an error :

    Command::cargo_bin("myTool")
        .unwrap()
        .arg("delete")
        .arg("resource")
		.assert()
		.success();

    Command::cargo_bin("myTool")
        .unwrap()
        .arg("read")
        .arg("resource")
		.assert()
		.failure();
}

However in some cases the "read" operation succeed because the resource was not yet deleted by the remote system.
I would need to check, because even if I was expecting a failure, a success might not invalidate the test.
Continuing the above example :

// first assert to delete the resource

// then read back: 

    let read = Command::cargo_bin("myTool")
        .unwrap()
        .arg("read")
        .arg("resource")
		.assert();

match read.try_failure() {
        Ok(assert) => {
				// here we are good but may want to verify a couple of things from the output
            	let output: &assert.get_output().stdout;
        }
        Err(err) => {
            let output: myType = serde_json::from_slice(&err.0.get_output().stdout).unwrap();
            // we check if it was marked for deletion. If so, it's all good.
            assert!(output.metadata.deletion_timestamp.is_some());
        }
    }

I was not sure if a getter method or simply marking the field public was the better choice there.. Let me know what you think.

[jbtrystram]

@epage Bumping this. Would you be interested in that change ?

[epage]

This is now available in v2.0.5