Add a test case for decode_slice_unchecked panic

[AbeZbm]

Hi,

Thanks for your time to review this PR.

By examining the existing code, we found that one test can be added to improve the repo's overall test coverage. We've added
one test for

rust-base64/src/engine/mod.rs

Lines 411 to 412 in 6243bd1

	DecodeSliceError::OutputSliceTooSmall => {
	panic!("Output slice is too small")

Thanks again for reviewing.

[AbeZbm]

@marshallpierce Hello! I was wondering if there's any feedback on this PR. I'm happy to address any concerns or make changes as needed. Thank you!