tests: rename to easily distinguish days

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/bin/day01.rs

@@ -59,7 +59,7 @@ fn main() -> Result<()> {
 }
 
 #[cfg(test)]
-mod tests {
+mod day_01 {
     use super::*;
 
     #[test]

src/bin/day02.rs

@@ -184,7 +184,7 @@ fn main() -> Result<()> {
 }
 
 #[cfg(test)]
-mod tests {
+mod day_02 {
     use super::*;
 
     #[test]

src/bin/day03.rs

@@ -96,7 +96,7 @@ fn main() -> Result<()> {
 }
 
 #[cfg(test)]
-mod tests {
+mod day_03 {
     use super::*;
 
     #[test]