because every test run use a different instance of the class, I think
we don't need to clear() here.

using clear causes:
Traceback (most recent call last):
  File "/srv/slapgrid/slappart7/t/ean/soft/3216c6675c8ec7c75ba48e3939c44a66/eggs/Zope-4.8.7-py3.9.egg/Testing/ZopeTestCase/base.py", line 100, in tearDown
    self.beforeTearDown()
  File "<portal_components/test.erp5.testTradeModelLine>", line 106, in beforeTearDown
  File "/srv/slapgrid/slappart7/t/ean/shared/python3/25c3b9e7a12e170bd030a475160ea994/lib/python3.9/_collections_abc.py", line 929, in clear
    self.popitem()
  File "/srv/slapgrid/slappart7/t/ean/shared/python3/25c3b9e7a12e170bd030a475160ea994/lib/python3.9/_collections_abc.py", line 918, in popitem
    key = next(iter(self))
  File "<portal_components/test.erp5.testTradeModelLine>", line 66, in __iter__
TypeError

... because unittest.TestCase should not have an __iter__ method