Added day 4.. early I know! And i fixed unit tests not compiling

src/solutions/day04.rs

@@ -0,0 +1,66 @@
+use super::Solution;
+
+pub struct Day04 {}
+
+impl Solution for Day04 {
+    fn part1(
+        &self,
+        _input: &mut Vec<String>,
+    ) -> Result<Box<dyn std::fmt::Display>, Box<dyn std::error::Error>> {
+        Ok(Box::new("Ready"))
+    }
+
+    fn part2(
+        &self,
+        _input: &mut Vec<String>,
+    ) -> Result<Box<dyn std::fmt::Display>, Box<dyn std::error::Error>> {
+        Ok(Box::new("Ready"))
+    }
+
+    fn get_day(&self) -> u8 {
+        4
+    }
+}
+
+impl Day04 {}
+
+/// Test from puzzle input
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::*;
+
+    #[test]
+    fn part1() {
+        let challenge = day04::Day04 {};
+
+        //Complete the Challenge
+        let answer = challenge
+            .part1(
+                utils::get_input(challenge.get_day(), utils::InputType::Test1)
+                    .unwrap()
+                    .as_mut(),
+            )
+            .unwrap()
+            .to_string();
+
+        assert_eq!(answer, "Ready");
+    }
+
+    #[test]
+    fn part2() {
+        let challenge = day04::Day04 {};
+
+        //Complete the Challenge
+        let answer = challenge
+            .part2(
+                utils::get_input(challenge.get_day(), utils::InputType::Test2)
+                    .unwrap()
+                    .as_mut(),
+            )
+            .unwrap()
+            .to_string();
+
+        assert_eq!(answer, "Ready");
+    }
+}