fix: name each exported addition's core

Batocera, ROCKNIX and Recalbox find a file through the core that wants
it. Additions now carry that core under the platform's own name: the
emulator/core pair checkBios reads (ROCKNIX's from its emulators
package), or Recalbox's core attribute. One with no nameable core, or
filed in a system gated on another core, is counted as not stated.

tests/test_export_counts.py

@@ -331,7 +331,8 @@ class OneMd5FormatsAddWhole(unittest.TestCase):
         from exporter.batocera_exporter import Exporter as Batocera  # noqa: PLC0415
 
         several = NativeFile("boot.bin", "boot.bin", "dc", truth={"md5": [A, B, C]})
-        single = NativeFile("boot2.bin", "boot2.bin", "dc", truth={"md5": A})
+        single = NativeFile("boot2.bin", "boot2.bin", "dc", truth={"md5": A},
+                          runners=[(False, "flycast")])
         self.assertFalse(Batocera.writable(several))
         self.assertTrue(Batocera.writable(single))