Tests: simplify test_settings maintenance

If no version-specific tests.test_settings.settings* file
exists for the current Django version, fall back to an
earlier settings file.

- runtests.py find latest settings_N_M earlier than or
  matching Django version N.M being tested, rather than
  requiring exact match
- remove test_settings files that were just duplicates
  of earlier versions

runtests.py

@@ -5,14 +5,53 @@
 # runtests.py [tests.test_x tests.test_y.SomeTestCase ...]
 
 import os
+import re
 import sys
 import warnings
+from pathlib import Path
 
 import django
 from django.conf import settings
 from django.test.utils import get_runner
 
 
+def find_test_settings():
+    """
+    Return dotted path to Django settings compatible with current Django version.
+
+    Finds highest tests.test_settings.settings_N_M.py where N.M is <= Django version.
+    (Generally, default Django settings don't change meaningfully between Django
+    releases, so this will fall back to the most recent settings when there isn't an
+    exact match for the current version, while allowing creation of new settings
+    files to test significant differences.)
+    """
+    django_version = django.VERSION[:2]  # (major, minor)
+    found_version = None  # (major, minor)
+    found_path = None
+
+    for settings_path in Path("tests/test_settings").glob("settings_*.py"):
+        try:
+            (major, minor) = re.match(
+                r"settings_(\d+)_(\d+)\.py", settings_path.name
+            ).groups()
+            settings_version = (int(major), int(minor))
+        except (AttributeError, TypeError, ValueError):
+            raise ValueError(
+                f"File '{settings_path!s}' doesn't match settings_N_M.py"
+            ) from None
+        if settings_version <= django_version:
+            if found_version is None or settings_version > found_version:
+                found_version = settings_version
+                found_path = settings_path
+
+    if found_path is None:
+        raise ValueError(f"No suitable test_settings for Django {django.__version__}")
+
+    # Convert Path("test/test_settings/settings_N_M.py")
+    # to dotted module "test.test_settings.settings_N_M":
+    return ".".join(found_path.with_suffix("").parts)
+
+
 def setup_and_run_tests(test_labels=None):
     """Discover and run project tests. Returns number of failures."""
     test_labels = test_labels or ["tests"]
@@ -32,9 +71,9 @@ def setup_and_run_tests(test_labels=None):
     # show DeprecationWarning and other default-ignored warnings:
     warnings.simplefilter("default")
 
-    os.environ["DJANGO_SETTINGS_MODULE"] = (
-        "tests.test_settings.settings_%d_%d" % django.VERSION[:2]
-    )
+    settings_module = find_test_settings()
+    print(f"Using settings module {settings_module!r}.")
+    os.environ["DJANGO_SETTINGS_MODULE"] = settings_module
     django.setup()
 
     TestRunner = get_runner(settings)