Make test fixtures return each recipe separately

[msimberg]

Makes each test parameterized over the recipe and easier to see which tests pass or fail. Each recipe becomes a separate test:

unittests/test_schema.py::test_config_yaml PASSED                                                                                                                                                                                  [  4%]
unittests/test_schema.py::test_recipe_config_yaml[host-recipe] PASSED                                                                                                                                                              [  8%]
unittests/test_schema.py::test_recipe_config_yaml[base-nvgpu] PASSED                                                                                                                                                               [ 13%]
unittests/test_schema.py::test_recipe_config_yaml[cache] PASSED                                                                                                                                                                    [ 17%]
unittests/test_schema.py::test_recipe_config_yaml[with-repo] PASSED                                                                                                                                                                [ 21%]
unittests/test_schema.py::test_recipe_config_yaml[with-multi-repos] PASSED                                                                                                                                                         [ 26%]
unittests/test_schema.py::test_compilers_yaml PASSED                                                                                                                                                                               [ 30%]
unittests/test_schema.py::test_recipe_compilers_yaml[host-recipe] PASSED                                                                                                                                                           [ 34%]
unittests/test_schema.py::test_recipe_compilers_yaml[base-nvgpu] PASSED                                                                                                                                                            [ 39%]
unittests/test_schema.py::test_recipe_compilers_yaml[cache] PASSED                                                                                                                                                                 [ 43%]
unittests/test_schema.py::test_recipe_compilers_yaml[with-repo] PASSED                                                                                                                                                             [ 47%]
unittests/test_schema.py::test_recipe_compilers_yaml[with-multi-repos] PASSED                                                                                                                                                      [ 52%]