Fix `payjoin-cli` v1 coverage

[spacebear21]

One thing I noticed is that payjoin-cli v1 is not covered, which is likely due to the missing _danger-local-https in contrib/coverage.sh for the v1 llvm-cov command. This can be addressed in a separate PR.

Originally posted by @spacebear21 in #497 (review)

Follow-up on #475 and #494

There is another issue, which is that payjoin-cli v1 does not handle SIGINT gracefully, so it still gets forcefully shutdown in the e2e tests despite using sigint.

We should add interrupt handlers to payjoin-cli v1 to fix this.

[spacebear21]

Reopened because payjoin-cli v1 still needs this to properly report coverage:

There is another issue, which is that payjoin-cli v1 does not handle SIGINT gracefully, so it still gets forcefully shutdown in the e2e tests despite using sigint.

We should add interrupt handlers to payjoin-cli v1 to fix this.

[nothingmuch]

Related issue: in #497 payjoin-cli resume tries to print after handling SIGINT, resulting in Broken pipe crash.

After sending sigint, child processes should have their stdout drained, and then waited on asserting that exit status was 0.