Remove accept_nonblock.rb, add test_integration_ssl.rb

[MSP-Greg]

Description

accept_nonblock.rb has existed since Puma's SSL implementation used Ruby OpenSSL classes and objects. It has since removed those, and only uses the OpenSSL libraries.

Remove the file, as it is no longer needed. It loaded openssl, and that is also unneeded on MRI Rubies.

Add a new 'ssl' test file that starts a Puma server using IO.popen, which decouples the CI environment and what it has loaded. The test is run on all platforms.

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