Add pretty printer test for async blocks

Currently fails with:

    ---- [pretty] pretty/async.rs stdout ----

    error: pretty-printed source does not match expected source
    expected:
    ------------------------------------------
    // pp-exact
    // pretty-compare-only
    // edition:2021

    async fn f() {
        let first = async { 1 };
        let second = async move { 2 };
        join(first, second).await
    }

    ------------------------------------------
    actual:
    ------------------------------------------
    // pp-exact
    // pretty-compare-only
    // edition:2021

    async fn f() {
        let first = async  { 1 };
        let second = async move  { 2 };
        join(first, second).await
    }

    ------------------------------------------
    diff:
    ------------------------------------------
    3	// edition:2021
    4
    5	async fn f() {
    -	    let first = async { 1 };
    -	    let second = async move { 2 };
    +	    let first = async  { 1 };
    +	    let second = async move  { 2 };
    8	    join(first, second).await
    9	}
    10

Author: dtolnay

src/test/pretty/async.rs

@@ -0,0 +1,9 @@
+// pp-exact
+// pretty-compare-only
+// edition:2021
+
+async fn f() {
+    let first = async { 1 };
+    let second = async move { 2 };
+    join(first, second).await
+}