test: add test for decoding variable-length inputs

src/decoder.nr

@@ -1069,3 +1069,28 @@ fn test_decode_with_padding() {
     ];
     assert(result == expected);
 }
+
+#[test]
+fn test_decode_var() {
+    // base64: SGVsbG8sIFdvcmxkIQ==
+    let input: BoundedVec<u8, 24> = BoundedVec::from_array([
+        83, 71, 86, 115, 98, 71, 56, 115, 73, 70, 100, 118, 99, 109, 120, 107, 73, 81, 61, 61,
+    ]);
+    // base64: SGVsbG8sIFdvcmxkIQ
+    let input_no_pad: BoundedVec<u8, 24> = BoundedVec::from_array([
+        83, 71, 86, 115, 98, 71, 56, 115, 73, 70, 100, 118, 99, 109, 120, 107, 73, 81,
+    ]);
+    // "Hello, World!"
+    let expected: BoundedVec<u8, 16> =
+        BoundedVec::from_array([72, 101, 108, 108, 111, 44, 32, 87, 111, 114, 108, 100, 33]);
+
+    // all configurations should decode the same way
+    let result = STANDARD.decode_var(input);
+    assert(result == expected);
+    let result = STANDARD_NO_PAD.decode_var(input_no_pad);
+    assert(result == expected);
+    let result = URL_SAFE_WITH_PAD.decode_var(input);
+    assert(result == expected);
+    let result = URL_SAFE.decode_var(input_no_pad);
+    assert(result == expected);
+}