Candid test suite: Write a few more spacebomb tests

[nomeata]

Just to test a few more corner cases and variants.

[nomeata]

The CI seems to fail for unrelated reasons?

    Checking pretty-hex v0.2.1
error: slow zero-filling initialization
   --> rust/candid/src/types/value.rs:455:17
    |
454 |                 let mut buf = Vec::new();
    |                               ---------- help: consider replacing this with: `vec![0; len]`
455 |                 buf.resize(len, 0);
    |                 ^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
    = note: `-D clippy::slow-vector-initialization` implied by `-D clippy::all`