test(integv2): fixes to allow test_record_padding to partially run

[johubertj]

Description of changes:

• Adding debugging statements that were used to help solve the issue
• Move the check for close marker from the end of all I/O operations to every individual iteration
• Change the assert statement to assert for a non zero value and check for stderr

Problem

The way the I/O loop previously operated is shown w/ the below psuedo code.

for file_descriptor in [stdout, stderr]:
   if file_descriptor.is_empty():
       continue
    
   data_str = file_descriptor.read()

if close_marker in data_str:
   done

For a test to exit successfully, we must detect the close marker from stdout. Previously stderr was always empty so data_str was always populated from stdout.

In PR #3945, s2n-tls behavior was changed so that improper shutdowns cause errors. This affected test_record_padding.py because stderr was no longer empty. While the close marker was still read from stdout into data_str, the second iteration of the I/O loop overwrote the close marker with the message from stderr.

This change solves this problem by checking for the close marker every iteration.

Related Issue

• IntegV2/test_record_padding: I/O issues case non zero exit sometimes #5130

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jmayclin]

Now that it's passing, let's go ahead and addtest_record_padding back to the CI:

s2n-tls/codebuild/spec/buildspec_ubuntu_integrationv2.yml

Lines 38 to 45 in 8201205

	INTEGV2_TEST:
	- "test_client_authentication test_dynamic_record_sizes test_sslyze test_sslv2_client_hello"
	- "test_happy_path"
	- "test_cross_compatibility"
	- "test_early_data test_well_known_endpoints test_hello_retry_requests test_sni_match test_pq_handshake test_fragmentation test_key_update"
	- "test_session_resumption test_renegotiate_apache test_buffered_send"
	- "test_npn test_signature_algorithms"
	- "test_version_negotiation test_external_psk test_ocsp test_renegotiate test_serialization"