Tests: check that home screen becomes black on sign

 - on preattestation, attestation, attestation-with-dal and block
 - currently, it is only effective on nanos

test/python/test_instructions.py

@@ -305,6 +305,8 @@ def test_sign_preattestation(
             f"Expected hash {preattestation.hash.hex()} but got {preattestation_hash.hex()}"
         account.check_signature(signature, bytes(preattestation))
 
+    tezos_navigator.assert_screen(Path(test_name) / "black_screen.png")
+
     tezos_navigator.check_app_context(
         account,
         chain_id=main_chain_id,
@@ -349,6 +351,8 @@ def test_sign_attestation(
             f"Expected hash {attestation.hash.hex()} but got {attestation_hash.hex()}"
         account.check_signature(signature, bytes(attestation))
 
+    tezos_navigator.assert_screen(Path(test_name) / "black_screen.png")
+
     tezos_navigator.check_app_context(
         account,
         chain_id=main_chain_id,
@@ -393,6 +397,8 @@ def test_sign_attestation_dal(
             f"Expected hash {attestation.hash.hex()} but got {attestation_hash.hex()}"
         account.check_signature(signature, bytes(attestation))
 
+    tezos_navigator.assert_screen(Path(test_name) / "black_screen.png")
+
     tezos_navigator.check_app_context(
         account,
         chain_id=main_chain_id,
@@ -439,6 +445,8 @@ def test_sign_block(
             f"Expected hash {block.hash.hex()} but got {block_hash.hex()}"
         account.check_signature(signature, bytes(block))
 
+    tezos_navigator.assert_screen(Path(test_name) / "black_screen.png")
+
     tezos_navigator.check_app_context(
         account,
         chain_id=main_chain_id,

test/python/utils/navigator.py

@@ -158,7 +158,7 @@ def _send_and_navigate(self,
             )
         )
 
-    def _assert_screen(self, path: Path) -> None:
+    def assert_screen(self, path: Path) -> None:
         golden_path = self._snapshots_dir / path
         if not golden_path.parent.is_dir() and self._golden_run:
             golden_path.parent.mkdir(parents=True)
@@ -211,27 +211,27 @@ def check_app_context(self,
                 [NavInsID.RIGHT_CLICK] * 2,
                 screen_change_before_first_instruction=False
             )
-            self._assert_screen(path / "chain_id.png")
+            self.assert_screen(path / "chain_id.png")
             self.backend.right_click()
             self.backend.wait_for_screen_change()
             if account is not None and self.firmware.device == "nanos":
-                self._assert_screen(path / "public_key_hash_1.png")
+                self.assert_screen(path / "public_key_hash_1.png")
                 self.backend.right_click()
                 self.backend.wait_for_screen_change()
-                self._assert_screen(path / "public_key_hash_2.png")
+                self.assert_screen(path / "public_key_hash_2.png")
             else:
-                self._assert_screen(path / "public_key_hash.png")
+                self.assert_screen(path / "public_key_hash.png")
             self.backend.right_click()
             self.backend.wait_for_screen_change()
-            self._assert_screen(path / "high_watermark.png")
+            self.assert_screen(path / "high_watermark.png")
             self.navigator.navigate(
                 [NavInsID.LEFT_CLICK] * 4,
                 screen_change_before_first_instruction=False
             )
         else:
             self.home.settings()
             self.backend.wait_for_screen_change()
-            self._assert_screen(path / "app_context.png")
+            self.assert_screen(path / "app_context.png")
             self.settings.multi_page_exit()
             self.backend.wait_for_screen_change()