Add test to cover when unable to replace magics (#2471)

Another follow-up from #2357, adding a test for uncovered code.

src/black/handle_ipynb_magics.py

@@ -53,6 +53,7 @@
         "%%writefile",
     )
 )
+TOKEN_HEX = secrets.token_hex
 
 
 @dataclasses.dataclass(frozen=True)
@@ -188,10 +189,10 @@ def get_token(src: str, magic: str) -> str:
     """
     assert magic
     nbytes = max(len(magic) // 2 - 1, 1)
-    token = secrets.token_hex(nbytes)
+    token = TOKEN_HEX(nbytes)
     counter = 0
-    while token in src:  # pragma: nocover
-        token = secrets.token_hex(nbytes)
+    while token in src:
+        token = TOKEN_HEX(nbytes)
         counter += 1
         if counter > 100:
             raise AssertionError(

tests/test_ipynb.py

@@ -453,3 +453,12 @@ def test_ipynb_and_pyi_flags() -> None:
     assert isinstance(result.exception, SystemExit)
     expected = "Cannot pass both `pyi` and `ipynb` flags!\n"
     assert result.output == expected
+
+
+def test_unable_to_replace_magics(monkeypatch: MonkeyPatch) -> None:
+    src = "%%time\na = 'foo'"
+    monkeypatch.setattr("black.handle_ipynb_magics.TOKEN_HEX", lambda _: "foo")
+    with pytest.raises(
+        AssertionError, match="Black was not able to replace IPython magic"
+    ):
+        format_cell(src, fast=True, mode=JUPYTER_MODE)