export.selection.withdefault and export.selection both have a selection field
with Foo and Bar as possible selections.
When doing
`self.env["base.update.translations"].create({'lang': 'fr_FR'}).act_update()`
the base.update.translations wizard exports the translations of ALL modules,
including the ones of the test_impex module.

The generate po entry is something like:
```
  #. modules: test_impex, test_translation_import
  #: selection:export.selection,value:0
  #: selection:export.selection.withdefault,value:0
  #: selection:test.translation.import,import_type:0
  msgid "Bar"
  msgstr "Bar in french"
```
but may also be
```
  #. modules: test_translation_import, test_impex
  #: selection:export.selection,value:0
  #: selection:export.selection.withdefault,value:0
  #: selection:test.translation.import,import_type:0
  msgid "Bar"
  msgstr "Bar in french"
```
(not sure where the indeterminism comes from)

which creates 3 translation entries that are sometimes linked to the module
test_impex (ignored when exporting the translations of test_translation_import)
but are sometimes linked to the module test_translation_import instead

In the second case, this makes 3 more translations that is was supposed to
exist.

This commit hides the symptom by making sure there is no longer a clash
when exporting the translations of the module test_translation_import.
A real fix to avoid creating unnecessary translations during an execution of
the base.update.translations wizard must also be made in the future.

It is assumed the unnecessary translations problem was already present before
a7621137cb0d75ec545152d5e1d413c9d3548e45 but polib introduced the indeterminism
making the test fails from time to time.