Put tests in sub module for template.

2021/src/template.rs

@@ -21,6 +21,10 @@ fn part2(depths: &[u32]) -> Result<u32> {
 }
 */
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+
     #[test]
     fn test_part1() -> Result<()> {
         let input = r#"
@@ -40,3 +44,4 @@ fn test_part2()->Result<()> {
     Ok(())
     }
     */
+}