fix: kill all server-related processes on test session end

[guilefoylegaurav]

Description

This PR fixes #249

Changes

• replace process.terminate() with os.killpg(). The former does not kill child processes spawned, while the latter does.
• scope pytest fixtures to session level
• use os.killpg() (POSIX) or process.kill() (non-POSIX) as per host OS

Documentation

• No documentation update is required

How I've tested my work

• Passed 25/25 tests on running pytest inside integration_tests/ (Debian 10)

• Pending:

  • Windows: Passed 23/25 tests on running pytest inside integration_tests/
    • Reason: On test_global_index_request and test_dev_index_request() python3 invoked in shell instead of python - for Windows versions of Python (i.e. not WSL), python3 is not typically a standard alias

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