tests: update tests faking stdin

since d7f80021 (Fix support for files containing non-ascii chars., 2024-06-12) we need sys.stdin which supports reconfigure.

Before this change, we had failures like this:

       ======================================================================
       ERROR: test_timestamp_mixed_in_timestamp (apachedex.tests.TestMalformedInput.test_timestamp_mixed_in_timestamp)
       ----------------------------------------------------------------------
       Traceback (most recent call last):
       File "/srv/slapgrid/slappart15/srv/project/apachedex/apachedex/tests.py", line 51, in test_timestamp_mixed_in_timestamp
       apachedex.main()
       File "/srv/slapgrid/slappart15/srv/project/apachedex/apachedex/__init__.py", line 1380, in wrapper
       return func(*args, **kw)
              ^^^^^^^^^^^^^^^^^
       File "/srv/slapgrid/slappart15/srv/project/apachedex/apachedex/__init__.py", line 1644, in main
       logfile.reconfigure(
       ^^^^^^^^^^^^^^^^^^^
       AttributeError: '_io.StringIO' object has no attribute 'reconfigure'