System test shows zero questions in Main

[trichoplax]

Describe the bug
Adding a system test which navigates to question_one (which is in the Main category) and from that page clicks on "Posts" to see the posts list for that category, results in zero posts being displayed. Since question_one is in the Main category, the list should show at least one question.

To Reproduce
Steps to reproduce the behavior:

1. In your local development environment, checkout the branch trichoplax/system-tests-omit-questions-from-list.
2. Run the system tests.
3. Notice the failed test.

In CI, you can see the failure in pull request #1774.

Expected behavior
A question that exists in the Main category should show up in the posts list for the Main category.

Screenshots
Since switching from CircleCI to CI through GitHub, it appears that screenshots are not being recorded for failed system tests. However, the failure message is the same, so it appears that behaviour is consistent between the CI system tests and my local development environment in Docker.

The screenshot associated with the local failure:

Additional context
Previously, system test results were inconsistent between the CI and my local development environment. This was reported as #1570 following experimentation with system tests in #1569 and appears to be fixed, in that this unexpected behaviour is at least consistent between local and CI.

To be sure that the missing questions were not due to changes made in that previous testing pull request, I've set up a new pull request #1774 which has no code changes other than the addition of a single new test, which demonstrates this problem.