Move resolver test helpers into test file

They probably don't need to be global, and this fixes complaints from
Flake8 for unused imports.

Author: uranusjr

tests/conftest.py

@@ -19,7 +19,6 @@
 from tests.lib import DATA_DIR, SRC_DIR, TestData
 from tests.lib.certs import make_tls_cert, serialize_cert, serialize_key
 from tests.lib.path import Path
-from tests.lib.pip_api_helpers import finder, preparer
 from tests.lib.scripttest import PipTestEnvironment
 from tests.lib.server import make_mock_server, server_running
 from tests.lib.venv import VirtualEnvironment

tests/lib/pip_api_helpers.py renamed to tests/unit/test_resolution_resolvelib.py

@@ -4,7 +4,7 @@
 from pip._internal.commands.install import InstallCommand
 from pip._internal.index.collector import LinkCollector
 from pip._internal.index.package_finder import PackageFinder
-from pip._internal.models.index import PyPI
+# from pip._internal.models.index import PyPI
 from pip._internal.models.search_scope import SearchScope
 from pip._internal.models.selection_prefs import SelectionPreferences
 from pip._internal.network.session import PipSession