Ensure no global state leaked for ActiveModel::SecurePassword and BCrypt::Engine.
Prevent global state leak in I18n tests

`I18n.backend.store_translations` stores translation for the given local
in memory, and therefore is a potential leak of global state.

If these tests in `translation_test.rb` get to run before
`test_errors_on_nested_attributes_expands_name` in `validation_test.rb`,
the latter will fail at L78, in which 'Replies name can't be blank' is
expected, but 'name default attribute can't be blank' is obtained. 'name
default attribute' is carried over from the translation tests.

This patch makes sure translations hash is always cleaned up after each
test.