Split staticfiles tests into multiple files

[Tim Graham]

Update ​an existing pull request per the comments.

[Moritz Sichert]

Added PR.
The tests are now structured as follows:

cases.py:

class BaseStaticFilesTestCase
class StaticFilesTestCase
class BaseCollectionTestCase
class CollectionTestCase
class TestDefaults
class TestNoFilesCreated
def hashed_file_path
class TestHashedFiles
class FinderTestCase

settings.py:

TEST_ROOT
TESTFILES_PATH
TEST_SETTINGS

test_finders.py:

class TestFileSystemFinder
class TestAppDirectoriesFinder
class TestDefaultStorageFinder
class TestMiscFinder

test_management.py:

class TestFindStatic
class TestConfiguration
class TestCollection
class TestCollectionClear
class TestCollectionExcludeNoDefaultIgnore
class TestCollectionDryRun
class TestCollectionFilesOverride
class TestCollectionOverwriteWarning
class TestCollectionNonLocalStorage
class TestCollectionLinks

test_storage.py:

class TestCollectionCachedStorage
class TestCollectionManifestStorage
class TestCollectionSimpleCachedStorage
class CustomStaticFilesStorage
class TestStaticFilePermissions

test_templatetags.py:

class TestTemplateTag

test_views.py:

class TestServeStatic
class TestServeDisabled
class TestServeStaticWithDefaultURL
class TestServeStaticWithURLHelper

[Tim Graham <timograham@…>]

In b35b43d:

Fixed #24982 -- Split staticfiles tests into multiple files