Correctly close app body for all code paths

[MSP-Greg]

Description

1. At present, some code paths will not close the app body. Add code to always hold a reference to the app body, so when code calling lowlevel_error is executed, the reference to the app body is maintained.

2. test_rack_server.rb - add test_hijack_body_close - test for above

3. Small refactor of test_puma_server.rb. Takes too damn long.

4. test_puma_server.rb - add test_lowlevel_error_body_close to verify body close call when lowlevel_error

Closes #2999.

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[MSP-Greg]

Added commit '[CI] Fix two intermittent failures?', see https://github.com/puma/puma/actions/runs/3348499190

[MSP-Greg]

One person stated that this PR fixed the issue re #2999, see comment.

Re #3004, as mentioned, testing with hotwired/turbo-rails, I may write a few more tests.

This will cause merge conflicts with #3004, so I'd like to merge this.

Thoughts?