fix: check monitor threads are still working after ibc tests

[jjyr]

Fix monitor silently crashed issue #330 (comment)

We noticed sometimes CI returns success but the monitor thread is crashed silently, which is unexpected.

This PR adds code to check monitor is working at the end of ibc tests, and returns an error if it isn't working.

We call subscribe function of ChainEndpoint to check the monitor thread, if the monitor is crashed, the subscribe will return a SendError error, we converted it to Error::Relayer.

[ashuralyk]

for just solving the throw of failures in monitor, this PR is supposed to merge

[ashuralyk]

@jjyr I noticed the test between Axon and Axon has failures but CI also passed, please see this:
https://github.com/synapseweb3/forcerelay/actions/runs/6309594521/job/17174053398?pr=334 (in Run IBC Tests)

[jjyr]

Details

The discussion is moved to #339

This PR should be merged and #339 is not introduced in this PR.