Tests: add tests for dgettext, dpgettext, dngettext, and dnpgettext functions

[sergiolpzgmz]

Address the TODO in Lib/test/test_gettext.py:13-15

# TODO:
#  - Add new tests, for example for "dgettext"
#  - Tests should have only one assert.

Following the TODO guidance, each test has only one assert.
These functions were not previously tested directly, only indirectly
through other test classes.

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