Add regression test for issue 113

This does not currently compile.

    error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
      --> tests/test_transparent.rs:63:14
       |
    63 |     #[derive(Error, Debug)]
       |              ^^^^^
       |
    note: first, the lifetime cannot outlive the lifetime `'a` as defined on the impl at 65:18...
      --> tests/test_transparent.rs:65:18
       |
    65 |     struct Error<'a> {
       |                  ^^
    note: ...so that the types are compatible
      --> tests/test_transparent.rs:63:14
       |
    63 |     #[derive(Error, Debug)]
       |              ^^^^^
       = note: expected `&test_non_static::ErrorKind<'_>`
                  found `&test_non_static::ErrorKind<'a>`
       = note: but, the lifetime must be valid for the static lifetime...
    note: ...so that the type `test_non_static::ErrorKind<'_>` will meet its required lifetime bounds
      --> tests/test_transparent.rs:63:14
       |
    63 |     #[derive(Error, Debug)]
       |              ^^^^^
       = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/test_transparent.rs

@@ -57,3 +57,24 @@ fn test_anyhow() {
     assert_eq!("outer", error.to_string());
     assert_eq!("inner", error.source().unwrap().to_string());
 }
+
+#[test]
+fn test_non_static() {
+    #[derive(Error, Debug)]
+    #[error(transparent)]
+    struct Error<'a> {
+        inner: ErrorKind<'a>,
+    }
+
+    #[derive(Error, Debug)]
+    enum ErrorKind<'a> {
+        #[error("unexpected token: {:?}", token)]
+        Unexpected { token: &'a str },
+    }
+
+    let error = Error {
+        inner: ErrorKind::Unexpected { token: "error" },
+    };
+    assert_eq!("unexpected token: \"error\"", error.to_string());
+    assert!(error.source().is_none());
+}