test_freeze_with_setuptools: use mocks

This makes it possible to test both branches on any Python version.

src/pip/_internal/commands/freeze.py

 import sys
 from optparse import Values
-from typing import List
+from typing import AbstractSet, List
 
 from pip._internal.cli import cmdoptions
 from pip._internal.cli.base_command import Command
@@ -8,10 +8,18 @@ from pip._internal.cli.status_codes import SUCCESS
 from pip._internal.operations.freeze import freeze
 from pip._internal.utils.compat import stdlib_pkgs
 
-DEV_PKGS = {"pip"}
 
-if sys.version_info < (3, 12):
-    DEV_PKGS |= {"setuptools", "distribute", "wheel"}
+def _should_suppress_build_backends() -> bool:
+    return sys.version_info < (3, 12)
+
+
+def _dev_pkgs() -> AbstractSet[str]:
+    pkgs = {"pip"}
+
+    if _should_suppress_build_backends():
+        pkgs |= {"setuptools", "distribute", "wheel"}
+
+    return pkgs
 
 
 class FreezeCommand(Command):
@@ -64,7 +72,7 @@ class FreezeCommand(Command):
             action="store_true",
             help=(
                 "Do not skip these packages in the output:"
-                " {}".format(", ".join(DEV_PKGS))
+                " {}".format(", ".join(_dev_pkgs()))
             ),
         )
         self.cmd_opts.add_option(
@@ -80,7 +88,7 @@ class FreezeCommand(Command):
     def run(self, options: Values, args: List[str]) -> int:
         skip = set(stdlib_pkgs)
         if not options.freeze_all:
-            skip.update(DEV_PKGS)
+            skip.update(_dev_pkgs())
 
         if options.excludes:
             skip.update(options.excludes)

tests/functional/test_freeze.py

@@ -98,18 +98,38 @@ def test_freeze_with_pip(script: PipTestEnvironment) -> None:
 def test_freeze_with_setuptools(script: PipTestEnvironment) -> None:
     """
     Test that pip shows setuptools only when --all is used
-    or Python version is >=3.12
+    or _should_suppress_build_backends() returns false
     """
 
-    result = script.pip("freeze")
-    if sys.version_info >= (3, 12):
-        assert "setuptools==" in result.stdout
-    else:
-        assert "setuptools==" not in result.stdout
-
     result = script.pip("freeze", "--all")
     assert "setuptools==" in result.stdout
 
+    (script.site_packages_path / "mock.pth").write_text("import mock\n")
+
+    (script.site_packages_path / "mock.py").write_text(
+        textwrap.dedent(
+            """\
+                import pip._internal.commands.freeze as freeze
+                freeze._should_suppress_build_backends = lambda: False
+            """
+        )
+    )
+
+    result = script.pip("freeze")
+    assert "setuptools==" in result.stdout
+
+    (script.site_packages_path / "mock.py").write_text(
+        textwrap.dedent(
+            """\
+                import pip._internal.commands.freeze as freeze
+                freeze._should_suppress_build_backends = lambda: True
+            """
+        )
+    )
+
+    result = script.pip("freeze")
+    assert "setuptools==" not in result.stdout
+
 
 def test_exclude_and_normalization(script: PipTestEnvironment, tmpdir: Path) -> None:
     req_path = wheel.make_wheel(name="Normalizable_Name", version="1.0").save_to_dir(