tests: net: sockets: tls_configurations: Use fixture for openssl

[rlubos]

Currently, creating and killing openssl process was part of the test. In result, if the test case failed for whatever reason, the openssl process would remain open, causing disruptions in consecutive test executions.

Fix that by defining openssl server instance as a pytest fixture. That way, openssl process will be terminated regardless of the test result.

[rlubos]

For additional context - this has been observed from time to time in CI, where it could be seen that openssl failed to initialize due to corresponding TCP port being already open:
https://github.com/zephyrproject-rtos/zephyr/actions/runs/14989510350/job/42109974066?pr=89609

ERROR test_app_vs_openssl:test_app_vs_openssl.py:65 b'40A7AAF1FE7E0000:error:80000062:system library:BIO_bind:Address already in use:../crypto/bio/bio_sock2.c:178:calling bind()\n'
ERROR test_app_vs_openssl:test_app_vs_openssl.py:65 b'40A7AAF1FE7E0000:error:10000075:BIO routines:BIO_bind:unable to bind socket:../crypto/bio/bio_sock2.c:180:\n'

Before this change, if the test failed, openssl instance would not be killed:

DEBUG: #: [00:00:00.010,002] tls_configuration_sample: Test FAILED
ERROR: Read from device timeout occurred
FAILED
------------------------------ live log teardown -------------------------------
DEBUG: Running subprocess finished with return code 0

openssl process was still running after that

By using fixture, the process is killed regardless of the test result:

DEBUG: #: [00:00:00.010,002] tls_configuration_sample: Test FAILED
ERROR: Read from device timeout occurred
FAILED
------------------------------ live log teardown -------------------------------
INFO: Kill server
DEBUG: Running subprocess finished with return code 0

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[valeriosetti]

@rlubos sorry for the late reply, but I was in vacation last week. Thanks for fixing this!