I manually executed the tests in my env. What I found is this [1]. In particular I would like to focus on this:
ResourceWarning: Enable tracemalloc to get the object allocation traceback
Exception ignored in: <function PipeMutex.__del__ at 0x7f88df98c8b0>
Traceback (most recent call last):
  File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_log/pipe_mutex.py", line 137, in __del__
    self.close()
  File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_log/pipe_mutex.py", line 116, in close
    if self.wfd is not None:
AttributeError: 'PipeMutex' object has no attribute 'wfd'


This is another log captured during the execution of the Neutron functional jobs:
    Traceback (most recent call last):                                                                                                                                                                                                                                                   
                                                                                                                                                                                                                                                                                         
      File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/fixture.py", line 196, in setUp                                                                                                                                                                 
    self._setUp()                                                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                                                                         
      File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/_fixtures/logger.py", line 107, in _setUp                                                                                                                                                       
                                                                                                                                                                                                                                                                                         
      File "/usr/lib/python3.8/logging/__init__.py", line 1057, in __init__                                                                                                                                                                                                              
                                                                                                                                                                                                                                                                                         
      File "/usr/lib/python3.8/logging/__init__.py", line 873, in __init__                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                         
      File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_log/pipe_mutex.py", line 142, in pipe_createLock                                                                                                                                                    
                                                                                                                                                                                                                                                                                         
      File "/opt/stack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_log/pipe_mutex.py", line 35, in __init__                                                                                                                                                            
                                                                                                                                                                                                                                                                                         
    OSError: [Errno 24] Too many open files   


The last error is basically the problem reported in the description, but this one in particular is when the PipeMutex class is opening the pipes.


[1]https://paste.opendev.org/show/bnny0x0QGt1OgVhypuyM/