[r4victor]

Tests now mirror the src/dstack layout. A package's __init__.py is tested in test_{package}.py, or in a test named after its single component (test_api_client.py, test_pipeline.py).

• Move API client tests under api/server/ and the run collection tests under api/_public/.
• Split request exclude tests in api/test_offers.py into core/compatibility/test_runs.py and test_gpus.py.
• Move the REST plugin tests to plugins/builtin/rest_plugin/test_plugin.py.
• Rename job pipeline tests to test_jobs_{running,submitted,terminating}.py and the test_instances/ and test_runs/ pipeline test packages to instances/ and runs/, with test_termination.py for runs renamed to test_terminating.py.
• Move fluentbit tests to services/logs/test_fluentbit.py and split services/test_logs.py into logs/test_filelog.py and logs/test_aws.py.
• Move cli/services/configurators/test_profile.py to cli/services/test_profile.py and the cli/commands/test_dstack.py tests into cli/test_main.py.
• Merge Lium volume tests into test_compute.py, using its autospec'd compute fixture, backend config tests into services/test_config.py, and gateway update tests into gateways/test_gateways.py.