Koobs if you can, it would be good to understand where the failure is. My guess is that Python doesn’t like running a non-ASCII filename. The following is hopefully a simplified version of the test_cmd_line_script test case:

import os, subprocess, sys

script_name = os.fsdecode(b'./\xE7w\xF0.py')
script_file = open(script_name, 'w', encoding='utf-8')
script_file.write('print(ascii(__file__))\n')
script_file.close()

cmd_line = [sys.executable, '-X', 'faulthandler', '-I', script_name]
env = os.environ.copy()
env['TERM'] = ''
proc = subprocess.Popen(cmd_line, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, env=env)
out, err = proc.communicate()
print(proc.returncode)  # Should be 0 but Free BSD has 1
print(repr(err))  # Error is about encoding 0xE7 with ASCII
print(repr(out))  # If executed, this would be the file name

Hopefully fixing the above problem will help with the test_readline failure. The readline test case does Readline (tab) completions involving non-ASCII text, and it seems that the Python completion routine is no longer being called.