`GLib-CRITICAL` in `send_event` unit tests on target

[apljungquist]

When running the tests with --test-threads=1 (e.g. cargo-acap-sdk test -- -p send_event) the tests emit suspicious GLib-CRITICAL records:

running 11 tests
test tests::can_declare_without_callback ... ok
test tests::can_send_and_receive_event ... ok
test tests::can_send_and_receive_stateful_boolean ... 
(process:31302): GLib-CRITICAL **: 18:00:28.200: g_hash_table_remove_internal: assertion 'hash_table != NULL' failed

(process:31302): GLib-CRITICAL **: 18:00:28.201: g_hash_table_destroy: assertion 'hash_table != NULL' failed
ok
test tests::get_boolean_propagates_none ... ok
test tests::get_boolean_works_with_all_value ... ok
test tests::get_double_propagates_none ... ok
test tests::get_double_works_with_nan ... ok
test tests::get_double_works_with_suspicious_values ... ok
test tests::get_integer_propagates_none ... ok
test tests::get_integer_works_with_suspicious_values ... ok
test tests::get_string_none ... ok

test result: ok. 11 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 10.11s

This appears to have been the case ever since can_send_and_receive_stateful_boolean was added. I think the reason I didn't notice at that time is that I was running the tests manually without the extra --num-threads argument, because I was using a VLT camera that wasn't set up with the expected SSH access