[test] Fix unittest_i18n to run properly with "pytest -t" (closes #5576169) When running "pytest -t", many tests modules will be imported and run within a single python process. unittest_i18n fails when run after tests such as unittest_webtest.py or unittest_httptest.py. The main reason is that unittest_i18n does various tricks to add a test cube to the search path, which badly fails when a previous test module has already loaded "stuff" (for lack of a better word). Instead, let's just call the 'i18ncube' command with subprocess. The change from logilab.common.testlib to unittest is not directly needed, but comes as a free cleanup with the above changes.

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