feat(tests): add silence_c_stderr to silence SoundSwallower in test s…

…uites

test/basic_test_case.py

@@ -1,7 +1,10 @@
 """Common base class for the ReadAlongs test suites"""
 
 import logging
+import os
+import sys
 import tempfile
+from contextlib import contextmanager
 from pathlib import Path
 from unittest import TestCase
 
@@ -65,3 +68,26 @@ def tearDown(self):
             # Some test cases can set the logging level to DEBUG when they pass
             # --debug to a CLI command, but don't let that affect subsequent tests.
             LOGGER.setLevel(logging.INFO)
+
+
+@contextmanager
+def silence_c_stderr():
+    """Capture stderr from C output, e.g., from SoundSwallower.
+
+    Note: to capture stderr for both C and Python code, combine this with
+    redirect_stderr(), but you must use capture_c_stderr() first:
+        with capture_c_stderr(), redirect_stderr(io.StringIO()):
+            # code
+
+    Loosely inspired by https://stackoverflow.com/a/24277852, but much simplified to
+    address our narrow needs, namely to silence stderr in a context manager.
+    """
+
+    stderr_fileno = sys.stderr.fileno()
+    stderr_save = os.dup(stderr_fileno)
+    stderr_fd = os.open(os.devnull, os.O_RDWR)
+    os.dup2(stderr_fd, stderr_fileno)
+    yield
+    os.dup2(stderr_save, stderr_fileno)
+    os.close(stderr_save)
+    os.close(stderr_fd)