EnableRetryWithBackoffTests is flaky

[snicoll]

EnableRetryWithBackoffTests.randomExponential:77 [Wrong periods: %s[1000, 1124]]
Expecting value to be true but was false

[artembilan]

According to the report it turns out that our random logic in the ExponentialRandomBackOffPolicy:

next = (long) (next * (1 + r.nextFloat() * (getMultiplier() - 1)));

Accidentally ended up with with 1.
Therefore 1000 * 1 = 1000 😄

Fixing test respectively to include that possibility.