Constant failure on ValidatorRecurringEnrollment test

[linked0]

ValidatorRecurringEnrolment test fails constantly on CI and even local machines. But the test successes sometimes.
It doesn't seem that the errors could be fixed just by revealing pre-images beyond the validator cycle. Here's some trial for revealing the preimages beyond the validator cycle, which solves the no preimage error but does not for getting to the expected height on the test. There's some point where we should look into the code about the network manager managing peers.

These are two kinds of errors.

Module tests failed: agora.test.ValidatorRecurringEnrollment
core.exception.AssertError@source/agora/test/Base.d(797): Check condition failed after timeout of 10 secs and 100 attempts: Client #3 (agora://10.0.0.3/) has no preimage for client #2 (agora://10.0.0.2/) at height 21

and

Module tests failed: agora.test.ValidatorRecurringEnrollment
core.exception.AssertError@source/agora/test/ValidatorRecurringEnrollment.d(134): Check condition failed after timeout of 10 secs and 100 attempts: Node 0 has block height 21. Expected: 22
----------------
source/agora/utils/Utility.d:146 void agora.utils.Utility.retryFor(lazy bool, core.time.Duration, lazy immutable(char)[], immutable(char)[], ulong) [0x5632b5e7afbf]
<OMITTED>
source/agora/test/ValidatorRecurringEnrollment.d:134 void agora.test.ValidatorRecurringEnrollment.__unittest_L85_C1() [0x5632b63d08ee]

[hewison-chris]

The first error above is fixed in #3153
Commit with fix 9768f70