IGNFrance tests: fix exactly_one warning

test/geocoders/ignfrance.py::IGNFranceTestCase::test_reverse_no_result
  <...>/geopy/test/geocoders/util.py:91: DeprecationWarning: IGNFrance.reverse: default value for `exactly_one` argument will become True in geopy 2.0. Specify `exactly_one=False` as the argument explicitly to get rid of this warning.

test/geocoders/ignfrance.py

@@ -103,7 +103,7 @@ def test_geocode_no_result(self):
     def test_reverse_no_result(self):
         self.reverse_run(
             # North Atlantic Ocean
-            {"query": (35.173809, -37.485351)},
+            {"query": (35.173809, -37.485351), "exactly_one": True},
             {},
             expect_failure=True
         )