Improve ef_tests crate

tests/ef_tests/src/error.rs

@@ -0,0 +1,9 @@
+#[derive(Debug, PartialEq, Clone)]
+pub enum Error {
+    /// The value in the test didn't match our value.
+    NotEqual(String),
+    /// The test specified a failure and we did not experience one.
+    DidntFail(String),
+    /// Failed to parse the test (internal error).
+    FailedToParseTest(String),
+}