Add ui test for writing anyhow instead of bail

Currently fails:

    Expected test case to fail to compile, but it succeeded.

Author: dtolnay

tests/ui/must-use.rs

@@ -0,0 +1,11 @@
+#![deny(unused_must_use)]
+
+use anyhow::anyhow;
+
+fn main() -> anyhow::Result<()> {
+    if true {
+        // meant to write bail!
+        anyhow!("it failed");
+    }
+    Ok(())
+}