fix: Resolves failing tests when CERTIFICATE_LINKEDIN setting is configured

[haftamuk]

In order to be able to share course certificate to LinkedIn, configure SOCIAL_SHARING_SETTINGS with a value True for CERTIFICATE_LINKEDIN entry. The setting is not available by default, but when used there were failing tests and this change ensures all tests pass.

Fixes: #37428 (comment)

Testing instructions

• Change configuration value of CERTIFICATE_LINKEDIN to True at /lms/envs/common.py and cms/envs/common.py
  run
  python -Wd -m pytest -p no:randomly --ds=lms.envs.test common/djangoapps/student/tests/
  See all tests should pass

• Change configuration value of CERTIFICATE_LINKEDIN to False at /lms/envs/common.py and cms/envs/common.py
  run
  python -Wd -m pytest -p no:randomly --ds=lms.envs.test common/djangoapps/student/tests/
  See all tests should pass

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[deborahgu]

one nit here, also, could you resolve the conflicts?

[deborahgu]

[nit] look at how other tests in this file use ddt.data. For a simple case like this, that can be a cleaner way to test the condition. For example, you can use

    @ddt.data(
        (True, True),
        (False, False),
    )
    @ddt.unpack
    def test_post_to_linkedin_visibility(self, certificate_linkedin_enabled, linkedin_button_visible):

and then you don't need to test both configurations separately; the test runner will verify both configurations for you.