test_tribler_main job failing because of GUI imports in the core

[devos50]

Our test_tribler_main job is failing because of a GUI import in the core, e.g., see here. This job is linked to a badge that we show on our project homepage.

==================================== ERRORS ====================================
_ ERROR collecting src/tribler-core/tribler_core/components/metadata_store/restapi/tests/test_search_endpoint.py _
ImportError while importing test module '/home/jenkins/workspace/Test_tribler_main/tribler/src/tribler-core/tribler_core/components/metadata_store/restapi/tests/test_search_endpoint.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
src/tribler-core/tribler_core/components/metadata_store/restapi/tests/test_search_endpoint.py:11: in <module>
    from tribler_gui.utilities import to_fts_query
E   ModuleNotFoundError: No module named 'tribler_gui'

We should consider moving the to_fts_query function to the tribler_common package.