The content library tests were failing to locate templates when
rendering XBlocks, but since these tests are skipped in CI, the issue
went undetected.
This fix is marked temporary because a proper fix involves adding
lms/templates to the cms.envs.test MAKO_TEMPLATE_DIRS_BASE list. This
was tried, and caused unrelated tests to fail, and so we took this
approach instead. See PR for full details.
