Deal with flaky tests

[jonathanmetzman]

I'd say deploys fail at least 10% of the time because of flake. This is very annoying. Example:

AssertionError: 1 != 0
    self.assertEqual(1, len(testcases))
  File "/workspace/clusterfuzz/src/clusterfuzz/_internal/tests/core/bot/tasks/corpus_pruning_task_test.py", line 472, in test_prune
Traceback (most recent call last):

We should either:

1. Fix these tests
2. Delete them.
3. Retry flaky tests.