run-tests tool broken with Python 3.14

When trying to run a test case with our run-tests tool, using Python 3.14, it fails with:

./support/testing/run-tests -o ../outputs/ -k tests.toolchain.test_external.TestExternalToolchainCtngMusl.test_run
17:21:53 TestExternalToolchainCtngMusl            Starting
E
======================================================================
ERROR: test_run (tests.toolchain.test_external.TestExternalToolchainCtngMusl.test_run)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/thomas/projets/buildroot/support/testing/infra/basetest.py", line 92, in setUp
    super(BRTest, self).setUp()
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/thomas/projets/buildroot/support/testing/infra/basetest.py", line 52, in setUp
    self.b.delete()
    ~~~~~~~~~~~~~^^
  File "/home/thomas/projets/buildroot/support/testing/infra/builder.py", line 116, in delete
    if os.path.exists(self.builddir):
       ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "<frozen genericpath>", line 19, in exists
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

----------------------------------------------------------------------
Ran 1 test in 0.073s

FAILED (errors=1)

The test class is instantiated twice (it was the case prior to Python 3.14 as well). However, since Python 3.14, the second instance doesn't get the class variables like BRConfigTest.outputdir = os.path.abspath(args.output) that are defined by run-tests.