"Sporadic" test failures in c8y tests

[matthiasbeyer]

Describe the bug

There's a "sporadic" test failure in mapper_publishes_software_update_request(). Unfortunately, the "sporadicness" of the test is that it failed two out of three times for PR #1283 - which is not sporadic at all.

Here's one of the runs of that PR: https://github.com/thin-edge/thin-edge.io/runs/7464719118?check_suite_focus=true#step:7:1243

This error should not happen.

I guess it happens because the in-process testing broker that is spun up for each test is not yet in a fully functional state but the test already tries to send data to it. Because of this, we get a connection reset.

OR it may be because some malformed MQTT paket is sent to the broker, as brokers disconnect on malformed pakets.

To Reproduce

Not sure.

[rina23q]

It's related to MQTT connection.

---- c8y::tests::mapper_publishes_software_update_request stdout ----
thread 'c8y::tests::mapper_publishes_software_update_request' panicked at 'called `Result::unwrap()` on an `Err` value: I/O: connection closed by peer
Caused by:
    connection closed by peer', crates/core/tedge_mapper/src/c8y/tests.rs:79:57

I'm not certainly sure, however, I feel sometimes GitHub hosted runner kills the connection and make tests failed.

For example, another sporadic failure I observed is also MQTT related, Connection Error I/O: Connection refused (os error 111). https://github.com/thin-edge/thin-edge.io/pull/1292#issuecomment-1193766699

[PradeepKiruvale]

From GH Actions, looks like this is happening on armv7 architecture.

[rina23q]

Another failure:

---- c8y::tests::c8y_mapper_child_alarm_mapping_to_smartrest stdout ----
thread 'c8y::tests::c8y_mapper_child_alarm_mapping_to_smartrest' panicked at 'called `Result::unwrap()` on an `Err` value: I/O: connection closed by peer

Caused by:
    connection closed by peer', crates/core/tedge_mapper/src/c8y/tests.rs:406:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[rina23q]

Resolved by #1567. It's the fix of test.