xfail documentation unit tests for view classes, refs #299

More documentation unit tests. These ones check that every single **View class
imported into the datasette/app.py module are covered by our documentation.

Just one problem: they aren't documented yet. So I'm using the xfail pytest
decorator to mark these tests as allowed-to-fail. When you run the test suite
you now get a report of how many views still need to be documented, but it
doesn't fail the tests.

The output looks something like this:

    $ pytest tests/test_docs.py
    collected 31 items

    tests/test_docs.py ..........................XXXxx.                [100%]

    ============ 26 passed, 2 xfailed, 3 xpassed in 1.06 seconds ============

Once I have documented all the views I will remove the xfail so any future
views that are added without documentation will cause a test failure.

We can detect that a view is documented by looking for ReST label in the docs,
for example:

    .. _IndexView:

Some view classes can be used to power multiple URLs - the JsonDataView class
for example is used to power /-/metadata and /-/config and /-/plugins

In this case, the second part of the label can indicate the variety of page, e.g:

    .. _JsonDataView_metadata:

The test will pass as long as there is at least one label that starts with
_JsonDataView.

tests/test_docs.py

@@ -9,6 +9,7 @@ import pytest
 import re
 
 docs_path = Path(__file__).parent.parent / 'docs'
+label_re = re.compile(r'\.\. _([^\s:]+):')
 
 
 def get_headings(filename, underline="-"):
@@ -17,6 +18,11 @@ def get_headings(filename, underline="-"):
     return set(heading_re.findall(markdown))
 
 
+def get_labels(filename):
+    markdown = (docs_path / filename).open().read()
+    return set(label_re.findall(markdown))
+
+
 @pytest.mark.parametrize('config', app.CONFIG_OPTIONS)
 def test_config_options_are_documented(config):
     assert config.name in get_headings("config.rst")
@@ -49,3 +55,20 @@ def test_plugin_hooks_are_documented(plugin):
         s.split("(")[0] for s in get_headings("plugins.rst", "~")
     ]
     assert plugin in headings
+
+
+@pytest.fixture
+def documented_views():
+    view_labels = set()
+    for filename in docs_path.glob("*.rst"):
+        for label in get_labels(filename):
+            first_word = label.split("_")[0]
+            if first_word.endswith("View"):
+                view_labels.add(first_word)
+    return view_labels
+
+
+@pytest.mark.xfail
+@pytest.mark.parametrize("view_class", [v for v in dir(app) if v.endswith("View")])
+def test_view_classes_are_documented(documented_views, view_class):
+    assert view_class in documented_views