Add test cases to reproduce issue #2343

[crazytonyli]

I don't want to add failing test cases to the repo. To avoid blocking development and/or CI, the new test cases passes when running cargo test, but fails when adding a specific env: ISSUE_2342=1 cargo test.

Here is an example to verify the tests locally:

// Pass
cargo test --package uniffi-fixture-ext-types-sub-lib

// Panic
ISSUE_2343=1 cargo test --package uniffi-fixture-ext-types-sub-lib

From the crash log:

thread 'thread '<unnamed><unnamed>' panicked at .../uniffi_core/src/ffi/foreigncallbacks.rs:32:18' panicked at :
.../uniffi_core/src/ffi/foreigncallbacks.rs:Foreign pointer not set.  This is likely a uniffi bug.32