Make CTest do not truncate pytest output

As per our exchanges - thank you @clemenci for the tip :+1.

See also gaudi/Gaudi#261.

Phys/FunTuple/tests/conftest.py

+###############################################################################
+# (c) Copyright 2023 CERN for the benefit of the LHCb Collaboration           #
+#                                                                             #
+# This software is distributed under the terms of the GNU General Public      #
+# Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
+#                                                                             #
+# In applying this licence, CERN does not waive the privileges and immunities #
+# granted to it by virtue of its status as an Intergovernmental Organization  #
+# or submit itself to any jurisdiction.                                       #
+###############################################################################
+def pytest_report_header(config):
+    """
+    Make sure CTest does not truncate the output.
+    """
+    return "CTEST_FULL_OUTPUT"