Fix flaky test TestProblemsetFilters::test_filters. Closes #440

[stopnoanime]

The problem

This test used to check for the presence of a problem by asserting that the response HTML contained the problem name.
However, the response also included an HTML form with a generated CSRF token, sometimes this token would contain the problem name (test problems have short names like aaa so it wasn't that rare).
This caused the test to randomly fail.

Solution

Now the test expliclitly searches for a HTML tag with the necessary problem name.

[twalen]

Looks Good. But I wonder how many the same kind of errors still remain in the code.