Add tests for error conversion

tests/fail_compat.rs

@@ -0,0 +1,35 @@
+#[macro_use]
+extern crate failure;
+
+use failure::Fail;
+
+fn return_failure() -> Result<(), failure::Error> {
+    #[derive(Fail, Debug)]
+    #[fail(display = "my error")]
+    struct MyError;
+
+    let err = MyError;
+    Err(err.into())
+}
+
+fn return_error() -> Result<(), Box<dyn std::error::Error>> {
+    return_failure()?;
+    Ok(())
+}
+
+fn return_error_send_sync() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    return_failure()?;
+    Ok(())
+}
+
+#[test]
+fn smoke_default_compat() {
+    let err = return_error();
+    assert!(err.is_err());
+}
+
+#[test]
+fn smoke_compat_send_sync() {
+    let err = return_error_send_sync();
+    assert!(err.is_err());
+}