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

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.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>