feat: fill standalone emulator folders

With no platform detected, a machine running standalone emulators (ES-DE with DuckStation and PCSX2) was offered RetroArch's folder. The installer now fills the emulator folders of the standalone copies list that exist here, from the EmuDeck then RetroArch file lists, asking first at a terminal.

tests/test_install.py

@@ -3891,6 +3891,39 @@ class StandaloneFoldersAreNamedWhenThereAreAny(unittest.TestCase, _LoopbackRun):
         self.assertIn("--standalone-copies", out)
 
 
+class StandaloneEmulatorsWithoutAPlatformAreServed(unittest.TestCase, _LoopbackRun):
+    """ES-DE with DuckStation and PCSX2 and no RetroArch: the run said 'No
+    supported platform detected' and offered RetroArch's folder, which no
+    emulator on the machine reads."""
+
+    def setUp(self):
+        self.env = self.serve(self)
+        (self.home / "ES-DE").mkdir()
+        self.duckstation = self.home / ".local" / "share" / "duckstation" / "bios"
+        self.duckstation.mkdir(parents=True)
+
+    def test_the_emulator_folder_is_filled(self):
+        proc = _install_run(self.env, "--no-input", "--dry-run")
+        self.assertEqual(proc.returncode, 0, proc.stdout + proc.stderr)
+        self.assertIn("Found standalone emulator folders:", proc.stdout)
+        self.assertIn(f"to {self.duckstation}:", proc.stdout)
+        self.assertNotIn("--platform retroarch --dest", proc.stdout)
+        proc = _install_run(self.env, "--no-input")
+        self.assertEqual(proc.returncode, 0, proc.stdout + proc.stderr)
+        names = sorted(path.name for path in self.duckstation.iterdir())
+        self.assertIn("scph5501.bin", names)
+        self.assertTrue(all(name.startswith("scph") for name in names), names)
+        proc = _install_run(self.env, "--no-input", "--check")
+        self.assertEqual(proc.returncode, 0, proc.stdout + proc.stderr)
+        self.assertIn("All files up to date.", proc.stdout)
+
+    def test_no_emulator_folder_keeps_the_hint(self):
+        self.duckstation.rmdir()
+        proc = _install_run(self.env, "--no-input")
+        self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
+        self.assertIn("--platform retroarch --dest", proc.stdout)
+
+
 class TheEmulatorNamesHaveTheirOwnListing(unittest.TestCase, _LoopbackRun):
     """--emulator nope said "--list-cores lists all 441" and --list-cores
     printed 189: the buildbot names --emulator accepts were listed nowhere."""