Start proposal for parametrize with fixtures

doc/en/parametrize_with_fixtures.rst

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+=========================
+Parametrize with fixtures
+=========================
+
+Problem
+-------
+
+As a user I have functional tests that I would like to run against various
+scenarios.
+
+In this particular example we want to generate a new project based on a
+cookiecutter template. We want to test default values but also data that
+emulates user input.
+
+- use default values
+- emulate user input
+  - specify 'author'
+  - specify 'project_slug'
+  - specify 'author' and 'project_slug'
+
+This is how a functional test could look like:
+
+.. code-block:: python
+
+    import pytest
+
+    @pytest.fixture
+    def default_context():
+        return {'extra_context': {}}
+
+
+    @pytest.fixture(params=[
+        {'author': 'alice'},
+        {'project_slug': 'helloworld'},
+        {'author': 'bob', 'project_slug': 'foobar'},
+    ])
+    def extra_context(request):
+        return {'extra_context': request.param}
+
+
+    @pytest.fixture(params=['default', 'extra'])
+    def context(request):
+        if request.param == 'default':
+            return request.getfuncargvalue('default_context')
+        else:
+            return request.getfuncargvalue('extra_context')
+
+
+    def test_generate_project(cookies, context):
+        """Call the cookiecutter API to generate a new project from a
+        template.
+        """
+        result = cookies.bake(extra_context=context)
+
+        assert result.exit_code == 0
+        assert result.exception is None
+        assert result.project.isdir()
+