Display the unexpected warning in tests, if any (#865)

* Raise on the first unexpected warning

* Ignore the DeprecationWarning about Validator.iter_errors

tests/test_cli.py

@@ -5,6 +5,7 @@
 import time
 import unittest.mock as mock
 from argparse import ArgumentTypeError
+from contextlib import contextmanager
 from io import StringIO
 from shutil import copyfile
 from subprocess import check_call
@@ -1130,6 +1131,22 @@ def test_sync_script_dotdot_folder_564(tmpdir):
     jupytext(["--sync", str(nb_file)])
 
 
+@contextmanager
+def no_warning():
+    with pytest.warns(None) as warnings:
+        yield
+
+    # There should be no warning
+    for record in warnings:
+        # Temporary exception for for this one, see #865
+        if (
+            "Passing a schema to Validator.iter_errors is deprecated "
+            "and will be removed in a future release" in str(record.message)
+        ):
+            continue
+        raise RuntimeError(record)
+
+
 def test_jupytext_to_file_emits_a_warning(tmpdir):
     """The user may type
         jupytext notebook.ipynb --to script.py
@@ -1141,11 +1158,9 @@ def test_jupytext_to_file_emits_a_warning(tmpdir):
     nb_file = tmpdir.join("notebook.ipynb")
     write(new_notebook(), str(nb_file))
 
-    with pytest.warns(None) as record:
+    with no_warning():
         jupytext(["notebook.ipynb", "-o", "script.py"])
 
-    assert len(record) == 0
-
     with pytest.warns(UserWarning, match="Maybe you want to use the '-o' option"):
         jupytext(["notebook.ipynb", "--to", "script.py"])
 
@@ -1164,11 +1179,9 @@ def test_jupytext_set_formats_file_gives_an_informative_error(tmpdir, cwd_tmpdir
     nb_file = tmpdir.join("notebook.ipynb")
     md_file.write("Some text")
 
-    with pytest.warns(None) as warnings:
+    with no_warning():
         jupytext(["--sync", "notebook.md"])
 
-    assert not warnings
-
     assert py_file.exists()
     assert nb_file.exists()