[v6.d REVIEW] Improve tests for Buf.append and ilk
[v6.d REVIEW] Improve tests for Buf.append and ilk

- Use more appropriate test routines
- Use more random item values (e.g. in case there's bug that just
    pushes .elems instead of the value itself)

Orig: https://github.com/perl6/roast/commit/a41c9f530131ca18d9586e0abc37ea9dc0581555

perl6/roast,dogbert17/roast,dogbert17/roast