Refactor live tests for speed

[pombredanne]

=========================================================================== slowest 20 durations ============================================================================
31.38s call     tests/test_api.py::test_api_with_requirement_file
22.70s call     tests/test_resolution.py::test_without_supported_wheels
16.58s call     tests/test_cli.py::test_pdt_output_with_frozen_requirements
13.41s call     tests/test_cli.py::test_pdt_output_with_pinned_requirements
12.69s call     tests/test_resolution.py::test_get_resolved_dependencies_with_flask_and_python_36
8.87s call     tests/test_cli.py::test_cli_with_azure_devops_with_python_313
8.73s call     tests/test_cli.py::test_cli_with_single_index_url_except_pypi_simple
8.09s call     tests/test_resolution.py::test_get_resolved_dependencies_with_flask_and_python_310
7.98s call     tests/test_resolution.py::test_get_resolved_dependencies_with_flask_and_python_310_windows
7.56s call     tests/test_cli.py::test_cli_with_azure_devops_with_python_310
7.29s call     tests/test_cli.py::test_cli_with_azure_devops_with_python_38
6.34s call     tests/test_cli.py::test_cli_with_pinned_requirements_file
3.54s call     tests/test_cli.py::test_cli_with_requirements_and_ignore_errors
2.83s call     tests/test_cli.py::test_cli_with_environment_marker_and_complex_ranges
2.83s call     tests/test_api.py::test_api_with_prefer_source
2.74s call     tests/test_api.py::test_api_with_specifier_pdt
2.73s call     tests/test_api.py::test_api_with_specifier
2.64s call     tests/test_api.py::test_api_with_python_311
2.62s call     tests/test_api.py::test_api_with_recursive_requirement_file
2.28s call     tests/test_resolution.py::test_get_resolved_dependencies_with_tilde_requirement_using_json_api

... we can do better!

[pombredanne]

We could use mocks but this would defeat the purpose of having live tests. The simpler way is to have a few controlled, small packages that we publish and that are used in tests.