test: Replace %+ format with explicit one

%+ is an extension that jiff doesn't support (if we still want to
convert parse_datetime in the future, and since these are tests,
it's probably better to use an explicit format.

src/items/mod.rs

@@ -518,7 +518,7 @@ mod tests {
         // https://github.com/uutils/coreutils/issues/5177
         assert_eq!(
             "2023-07-27T13:53:54+00:00",
-            test_eq_fmt("%+", "@1690466034")
+            test_eq_fmt("%Y-%m-%dT%H:%M:%S%:z", "@1690466034")
         );
 
         // https://github.com/uutils/coreutils/issues/6398

src/lib.rs

@@ -666,7 +666,7 @@ mod tests {
             assert_eq!(
                 parse_datetime("28 feb 2023 + 1 day")
                     .unwrap()
-                    .format("%+")
+                    .format("%Y-%m-%dT%H:%M:%S%:z")
                     .to_string(),
                 "2023-03-01T00:00:00+00:00"
             );
@@ -678,7 +678,7 @@ mod tests {
             assert_eq!(
                 parse_datetime("2024-01-31 + 1 month")
                     .unwrap()
-                    .format("%+")
+                    .format("%Y-%m-%dT%H:%M:%S%:z")
                     .to_string(),
                 "2024-03-02T00:00:00+00:00",
             );
@@ -686,7 +686,7 @@ mod tests {
             assert_eq!(
                 parse_datetime("2024-02-29 + 1 month")
                     .unwrap()
-                    .format("%+")
+                    .format("%Y-%m-%dT%H:%M:%S%:z")
                     .to_string(),
                 "2024-03-29T00:00:00+00:00",
             );