Work around test suite warning in rustc older than 1.39

Apparently this is fine in 1.39 and up, but older compilers warn.

    warning: enum is never used: `Error`
       --> tests/test_display.rs:151:5
        |
    151 |     pub enum Error {}
        |     ^^^^^^^^^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

    warning: struct is never constructed: `MyError`
      --> tests/test_lints.rs:15:5
       |
    15 |     pub struct MyError;
       |     ^^^^^^^^^^^^^^^^^^^
       |
       = note: `#[warn(dead_code)]` on by default

tests/test_display.rs

@@ -149,6 +149,8 @@ fn test_void() {
     #[derive(Error, Debug)]
     #[error("...")]
     pub enum Error {}
+
+    let _: Error;
 }
 
 #[test]

tests/test_lints.rs

@@ -13,4 +13,6 @@ fn test_unused_qualifications() {
     #[derive(Debug, Error)]
     #[error("...")]
     pub struct MyError;
+
+    let _: MyError;
 }