Ensure --help examples in docs are always up to date, closes #336

Unit tests now check that docs/*.txt help examples are all up-to-date.

I ran into a problem here in that the terminal_width needed to be more
accurately defined - so I replaced update-docs-help.sh with update-docs-
help.py which hard-codes the terminal width.

tests/test_docs.py

@@ -1,12 +1,15 @@
 """
 Tests to ensure certain things are documented.
 """
+from click.testing import CliRunner
 from datasette import app
+from datasette.cli import cli
 from pathlib import Path
 import pytest
 import re
 
-markdown = (Path(__file__).parent.parent / 'docs' / 'config.rst').open().read()
+docs_path = Path(__file__).parent.parent / 'docs'
+markdown = (docs_path / 'config.rst').open().read()
 setting_heading_re = re.compile(r'(\w+)\n\-+\n')
 setting_headings = set(setting_heading_re.findall(markdown))
 
@@ -14,3 +17,25 @@ setting_headings = set(setting_heading_re.findall(markdown))
 @pytest.mark.parametrize('config', app.CONFIG_OPTIONS)
 def test_config_options_are_documented(config):
     assert config.name in setting_headings
+
+
+@pytest.mark.parametrize('name,filename', (
+    ('serve', 'datasette-serve-help.txt'),
+    ('package', 'datasette-package-help.txt'),
+    ('publish', 'datasette-publish-help.txt'),
+))
+def test_help_includes(name, filename):
+    expected = open(docs_path / filename).read()
+    runner = CliRunner()
+    result = runner.invoke(cli, [name, '--help'], terminal_width=88)
+    actual = '$ datasette {} --help\n\n{}'.format(
+        name, result.output
+    )
+    # actual has "Usage: cli package [OPTIONS] FILES"
+    # because it doesn't know that cli will be aliased to datasette
+    expected = expected.replace('Usage: datasette', 'Usage: cli')
+    print('expected')
+    print(expected)
+    print('actual')
+    print(actual)
+    assert expected == actual