Resolve "Implement a testing harness for functional tests"

This MR adds a Python-based testing harness that helps writing functional/integration tests. The harness provides common functionalities such as creating a workspace for tests, starting a GekkoFS server, and running client processes with an appropriate environment (e.g. LD_PRELOAD, LD_LIBRARY_PATH etc.), among others. Tests are automatically found by the harness and return easy to interpret error codes (e.g. success/failure). More details on how the harness work can be found in the wiki.

• Make tests self-contained
• Generate detailed logs per-test
• Allow using installed binaries to reduced CI artifact size
• Log asserts
• Add support for filtering out syscalls (e.g. epoll_wait()) in client logs (can be solved using lnav)
• Add support for multiple client log files for complex tests (will solve in future MR)
• Add support defining the network interface for tests via CMake option
• Clean up .gitlab-ci.yml

Closes #76 (closed)