Improve stability of test_studio_outline tests.

STUD-2084

@jzoldak and @dan-f please review

In addition to checking for the "add components" buttons to be present for STUD-2084, this includes a fix for a case with the "Loading" indicator still showing on the course overview page--

  File "/usr/lib/python2.7/unittest/case.py", line 327, in run
    testMethod()
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/2/TEST_SUITE/bok-choy/common/test/acceptance/tests/test_studio_outline.py", line 196, in test_unreleased_unpublished_changes_unlocked
    self.FUTURE_UNPUBLISHED_WARNING
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/2/TEST_SUITE/bok-choy/common/test/acceptance/tests/test_studio_outline.py", line 218, in _verify_unit_warning
    self._ensure_unit_present(unit_state)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/2/TEST_SUITE/bok-choy/common/test/acceptance/tests/test_studio_outline.py", line 243, in _ensure_unit_present
    subsection = self.course_outline_page.section(name).subsection(name)
  File "/home/jenkins/edx-venv/src/bok-choy/bok_choy/page_object.py", line 64, in wrapper
    return method(self, *args, **kwargs)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/2/TEST_SUITE/bok-choy/common/test/acceptance/pages/studio/overview.py", line 361, in section
    return self.child(title)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/2/TEST_SUITE/bok-choy/common/test/acceptance/pages/studio/overview.py", line 158, in child
    ).attrs('data-locator')[0]