Provide friendlier error if a file path is passed as a test label when running tests

[Chris Jerdonek]

Currently, if a user passes a file path as a test label to DiscoverRunner, they will get a somewhat unfriendly error that looks something like the following:

$ ./runtests.py test_runner/tests.py 
Testing against Django installed in '.../django/django' with up to 8 processes
Traceback (most recent call last):
  File "./runtests.py", line 593, in <module>
    options.timing,
  File "./runtests.py", line 325, in django_tests
    failures = test_runner.run_tests(test_labels or get_installed())
  File ".../django/django/test/runner.py", line 721, in run_tests
    suite = self.build_suite(test_labels, extra_tests)
  File ".../django/django/test/runner.py", line 612, in build_suite
    suite.addTests(tests)
  File ".../unittest/suite.py", line 57, in addTests
    for test in tests:
TypeError: 'NoneType' object is not iterable

This is because DiscoverRunner.build_suite() starts out with tests = None in each iteration of the loop and doesn't have an else clause for the file path case:
​https://github.com/django/django/blob/6f5dbe9dbe45b23b3befe4f1cd2ea13b6049ab96/django/test/runner.py#L574-L600

I imagine this is a common error because the help string tells the user to use "paths":

"Optional path(s) to test modules; e.g. "i18n" or "i18n.tests.TranslationTests.test_lazy_objects".

but without saying that file paths aren't allowed.

A more friendly error message could tell the user that they provided a path to a file, but that only dotted module paths and directory paths are supported.

[Chris Jerdonek]

Another option would be to support passing file paths, but that could be done separately, after first doing the easier thing of providing a nicer error message.

[Chris Jerdonek]

I would suggest that this not be done until after #32540, since it affects the if logic.

[Chris Jerdonek]

PR: ​https://github.com/django/django/pull/14177

[Mariusz Felisiak <felisiak.mariusz@…>]

In 235d23cf:

Refs #32532 -- Added DiscoverRunner.load_tests_for_label().

[Chris Jerdonek]

PR: ​https://github.com/django/django/pull/14180

[Chris Jerdonek]

With my posted patch, the new error will look like this:

$ ./runtests.py test_runner/tests.py
Testing against Django installed in '.../django/django' with up to 8 processes
Traceback (most recent call last):
  ...
RuntimeError: One of the test labels is a path to a file: 'test_runner/tests.py'
Test labels that are paths but not importable as names can only be directories.

(By the way, I unchecked "easy" because this wasn't necessarily so easy given that is_discoverable() had to be broken up to get the info needed for the patch.)

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0af81b2:

Refs #32532 -- Replaced is_discoverable() with try_importing().

[Mariusz Felisiak <felisiak.mariusz@…>]

In a89e975c:

Fixed #32532 -- Made DiscoverRunner raise RuntimeError when a test label is a file path.