feat(common): introduce test_utils::Dummy trait

A trait similar to the standard `Default` but dedicated to dummy test
double creation.

Author: Alenar

mithril-common/src/test_utils/double/mod.rs

@@ -0,0 +1,33 @@
+//! Test doubles
+//!
+//! Enable unit testing with controlled inputs and predictable behavior.
+
+/// A trait for giving a type a dummy value.
+///
+/// Sometimes in tests you need to provide a value for a type, but the actual value doesn't matter.
+/// This trait allows defining a "dummy" value for a type, separated from an eventual default
+/// value, that can be reused across multiple tests.
+///
+/// Note: should not be confused with "fake" values, fake values aim to be believable and contain
+/// valid cryptography (if they have some), dummies don't aim for those characteristics.
+///
+/// # Example
+/// ```
+/// use mithril_common::test_utils::double::Dummy;
+///
+/// struct MyType(String);
+///
+/// impl Dummy for MyType {
+///     fn dummy() -> Self {
+///         MyType("whatever".to_string())
+///     }
+/// }
+///
+/// let instance = MyType::dummy();
+/// ```
+pub trait Dummy: Sized {
+    /// Return a dummy value for the type
+    ///
+    /// Useful for test contexts when the actual value doesn't matter.
+    fn dummy() -> Self;
+}

mithril-common/src/test_utils/mod.rs

@@ -6,6 +6,7 @@
 //! * A builder of [MithrilFixture] to generate signers alongside a stake distribution
 //!
 
+pub mod double;
 pub mod fake_data;
 pub mod fake_keys;
 pub mod mock_extensions;