Fix some more error branches, assert sanity, and automatically test it all.

[riastradh-brave]

fix #14630

Substantive changes:

• Eliminate an assertion that socket closure happens more promptly than it is guaranteed to.
• Allow destruction functions to be idempotent, now that I've convinced myself that this is sensible because there are legitimately multiple paths into them (in asynchronous error events and in synchronous callbacks).

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

#14630

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

checked manual test plan (worked) and int tests passed

[diracdeltas]

master: a742002
0.24.x: 73ec80d
0.23.x: febd810