Add invoke task to run tests

This way, instead of needing to know that you run "pytest" and knowing
tricks like "pytest -m ''" to run webtests and HTML validation, you can
now just run "invoke test", with more intuitive flags. This also reduces
the output in quiet mode even more.

After adding invoke tasks for some of the other tools/checks, I'll be
able to switch the git hooks to use these instead.

tildes/pytest.ini

@@ -1,6 +1,6 @@
 [pytest]
 testpaths = tests
-addopts = -p no:cacheprovider --strict-markers -m "not (html_validation or webtest)"
+addopts = -p no:cacheprovider --strict-markers
 filterwarnings =
     ignore::DeprecationWarning
     ignore::PendingDeprecationWarning

tildes/tasks.py

@@ -9,6 +9,53 @@ from pathlib import Path
 from invoke import task
 
 
+def output(string):
+    """Output a string without a line ending and flush immediately."""
+    print(string, end="", flush=True)
+
+
+@task(
+    help={
+        "html-validation": "Include HTML validation (very slow, includes webtests)",
+        "quiet": "Reduce verbosity",
+        "webtests": "Include webtests (a little slow)",
+    }
+)
+def test(context, quiet=False, webtests=False, html_validation=False):
+    """Run the tests.
+
+    By default, webtests (ones that make actual HTTP requests to the app) and HTML
+    validation tests (checking the validity of the HTML on some of the site's pages) are
+    not run because they are slow, but you can include them with the appropriate flag.
+    """
+    # webtests are required as part of HTML validation
+    if html_validation:
+        webtests = True
+
+    pytest_args = []
+    excluded_markers = []
+
+    if not webtests:
+        excluded_markers.append("webtest")
+    if not html_validation:
+        excluded_markers.append("html_validation")
+
+    if excluded_markers:
+        excluded_marker_str = " or ".join(excluded_markers)
+        pytest_args.append(f'-m "not ({excluded_marker_str})"')
+
+    if quiet:
+        output("Running tests... ")
+
+        pytest_args.append("-q")
+        result = context.run("pytest " + " ".join(pytest_args), hide=True)
+
+        # only output the final line of pytest's stdout (test count + runtime)
+        print(result.stdout.splitlines()[-1])
+    else:
+        context.run("pytest " + " ".join(pytest_args), pty=True)
+
+
 @task
 def update_pip_requirements(context):
     """Use pip-tools to update package versions in the requirements files."""