Make expected result string in unit tests more readable

[WinkerDu]

Which issue does this PR close?

Closes #2412 .

Rationale for this change

Some unit tests in DataFusion organize expected result string vector in one line, like sql::union::union_distinct do

#[tokio::test]
async fn union_distinct() -> Result<()> {
    let ctx = SessionContext::new();
    let sql = "SELECT 1 as x UNION SELECT 1 as x";
    let actual = execute_to_batches(&ctx, sql).await;
    let expected = vec!["+---+", "| x |", "+---+", "| 1 |", "+---+"];
    assert_batches_eq!(expected, &actual);
    Ok(())
}

We can separate the expected result string vector into multiple line to make it more readable and easier to follow, by adding a #[rustfmt::skip] tag to skip fmt check as following code

#[tokio::test]
async fn union_distinct() -> Result<()> {
    let ctx = SessionContext::new();
    let sql = "SELECT 1 as x UNION SELECT 1 as x";
    let actual = execute_to_batches(&ctx, sql).await;
    #[rustfmt::skip]
    let expected = vec![
        "+---+",
        "| x |",
        "+---+",
        "| 1 |",
        "+---+"
    ];
    assert_batches_eq!(expected, &actual);
    Ok(())
}

What changes are included in this PR?

• Separates the expected result string vector into multiple line
• adds a #[rustfmt::skip] tag to skip fmt check

Are there any user-facing changes?

No.

[andygrove]

That is much easier to comprehend, especially when doing PR reviews. Thanks @WinkerDu.