Fixes flaky TestBlocksFetcher_nonSkippedSlotAfter test

[farazdagi]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• Since non-skipped slot is located using random sampling of future epochs, number of rounds in non-deterministic (normally 100 rounds cap is more than enough, but from time to time this is not enough -- and test fails).
• Instead of counting rounds, 5 seconds time cap is used (normally, completes in less than a second, occasionally spiking to 2 seconds). I've run the test for 1000 rounds, several times (with previous code ~ 1 out of every 20 tests were failing, with new time cap -- no failures were produced, so we are more robust now).

Which issues(s) does this PR fix?

Fixes #8201

Other notes for review