Stabilise iOS unit tests

[YuriyVelichkoPI]

Motivation

Save Circle CI resources on re-running failing tests.

Objectives

CircleCI reports that the most credits in the Prebid account we spent on iOS pipelines. On the one hand, it makes sense because of the price of iOS runners, on the other hand, we do too many re-runs due to the unstable test.

In the scope of this task, we should improve the test's stability so we don't re-run them almost every time.

Note: It is not the first time when we face this problem. If we can't stabilize some particular tests, we can consider the separation of testing flows. For instance, we can exclude the unstable tests from PR builds and run them only for release candidates builds or night builds.