feat: dump_settings management command (#36162)

This command dumps the current Django settings to JSON for
debugging/diagnostics. The output of this command is for *humans*... it
is NOT suitable for consumption by production systems.

In particular, we are introducing this command as part of a series of
refactorings to the Django settings files lms/envs/* and cms/envs/*.
We want to ensure that these refactorings do not introduce any
unexpected breaking changes, so the dump_settings command will both help
us manually verify our refactorings and help operators verify that our
refactorings behave expectedly when using their custom python/yaml
settings files.

Related to: https://github.com/openedx/edx-platform/pull/36131

openedx/core/djangoapps/util/tests/test_dump_settings.py

@@ -0,0 +1,64 @@
+"""
+Basic tests for dump_settings management command.
+
+These are moreso testing that dump_settings works, less-so testing anything about the Django
+settings files themselves. Remember that tests only run with (lms,cms)/envs/test.py,
+which are based on (lms,cms)/envs/common.py, so these tests will not execute any of the
+YAML-loading or post-processing defined in (lms,cms)/envs/production.py.
+"""
+import json
+
+from django.core.management import call_command
+
+from openedx.core.djangolib.testing.utils import skip_unless_lms, skip_unless_cms
+
+
+@skip_unless_lms
+def test_for_lms_settings(capsys):
+    """
+    Ensure LMS's test settings can be dumped, and sanity-check them for certain values.
+    """
+    dump = _get_settings_dump(capsys)
+
+    # Check: something LMS-specific
+    assert dump['MODULESTORE_BRANCH'] == "published-only"
+
+    # Check: tuples are converted to lists
+    assert isinstance(dump['XBLOCK_MIXINS'], list)
+
+    # Check: objects (like classes) are repr'd
+    assert "<class 'xmodule.x_module.XModuleMixin'>" in dump['XBLOCK_MIXINS']
+
+    # Check: nested dictionaries come through OK, and int'l strings are just strings
+    assert dump['COURSE_ENROLLMENT_MODES']['audit']['display_name'] == "Audit"
+
+
+@skip_unless_cms
+def test_for_cms_settings(capsys):
+    """
+    Ensure CMS's test settings can be dumped, and sanity-check them for certain values.
+    """
+    dump = _get_settings_dump(capsys)
+
+    # Check: something CMS-specific
+    assert dump['MODULESTORE_BRANCH'] == "draft-preferred"
+
+    # Check: tuples are converted to lists
+    assert isinstance(dump['XBLOCK_MIXINS'], list)
+
+    # Check: objects (like classes) are repr'd
+    assert "<class 'xmodule.x_module.XModuleMixin'>" in dump['XBLOCK_MIXINS']
+
+    # Check: nested dictionaries come through OK, and int'l strings are just strings
+    assert dump['COURSE_ENROLLMENT_MODES']['audit']['display_name'] == "Audit"
+
+
+def _get_settings_dump(captured_sys):
+    """
+    Call dump_settings, ensure no error output, and return parsed JSON.
+    """
+    call_command('dump_settings')
+    out, err = captured_sys.readouterr()
+    assert out
+    assert not err
+    return json.loads(out)