Tests/conftest.py: disable fontTools.configLogger globally during tests

otherwise it causes sides effects since logging state is global and should only
be done when __name__ == '__main__'.

We can capture logging messages via the caplog pytest fixture

Tests/conftest.py

@@ -0,0 +1,28 @@
+import fontTools
+import pytest
+
+
+@pytest.fixture(autouse=True, scope="session")
+def disableConfigLogger():
+    """Session-scoped fixture to make fontTools.configLogger function no-op.
+
+    Logging in python maintain a global state. When in the tests we call a main()
+    function from modules subset or ttx, a call to configLogger is made that modifies
+    this global state (to configures a handler for the fontTools logger).
+    To prevent that, we monkey-patch the `configLogger` attribute of the `fontTools`
+    module (the one used in the scripts main() functions) so that it does nothing,
+    to avoid any side effects.
+
+    NOTE: `fontTools.configLogger` is only an alias for the configLogger function in
+    `fontTools.misc.loggingTools` module; the original function is not modified.
+    """
+
+    def noop(*args, **kwargs):
+        return
+
+    originalConfigLogger = fontTools.configLogger
+    fontTools.configLogger = noop
+    try:
+        yield
+    finally:
+        fontTools.configLogger = originalConfigLogger