Add test case for hash calculations

While replacing the implementation of sha256, I noticed that there
is no test case actually calling the hash calculations.

Added two such test cases. Please note that I didn't verify that the
result is correct, but just took the value the existing implementation
returned. So those tests will only catch future regressions.

src/config.rs

@@ -170,4 +170,12 @@ mod test {
         assert_eq!("ask".parse::<InputSalt>().unwrap(), InputSalt::AskPassword);
         assert_eq!("lol".parse::<InputSalt>().unwrap(), InputSalt::default());
     }
+
+    #[test]
+    fn input_salt_obtain() {
+        assert_eq!(
+            InputSalt::String("abc".into()).obtain(&PasswordHelper::Stdin).unwrap(),
+            [186, 120, 22, 191, 143, 1, 207, 234, 65, 65, 64, 222, 93, 174, 34, 35, 176, 3, 97, 163, 150, 23, 122, 156, 180, 16, 255, 97, 242, 0, 21, 173]
+        )
+    }
 }