tests: work around non-thread-safeness of sysconfig.get_config_var()

tests/run-tests.py

@@ -1000,6 +1000,10 b' def killdaemons(pidfile):'
     return killmod.killdaemons(pidfile, tryhard=False, remove=True, logfn=vlog)
 
 
+# sysconfig is not thread-safe (https://github.com/python/cpython/issues/92452)
+sysconfiglock = threading.Lock()
+
+
 class Test(unittest.TestCase):
     """Encapsulates a single, runnable test.
 
@@ -1430,6 +1434,7 b' class Test(unittest.TestCase):'
             env["HGPORT%s" % offset] = '%s' % (self._startport + i)
 
         env = os.environ.copy()
+        with sysconfiglock:
         env['PYTHONUSERBASE'] = sysconfig.get_config_var('userbase') or ''
         env['HGEMITWARNINGS'] = '1'
         env['TESTTMP'] = _bytes2sys(self._testtmp)