Fixed flaky test in DiscoveryFactoryTest.java

[KellyShao]

Fixed flaky tests with error msg related to org.mockito.exceptions.verification.TooManyActualInvocations that caused by
verify(runtimeContext).isDisablePeerDiscovery(); in class com.quorum.tessera.discovery.DiscoveryFactoryTest.

• Error msg:

  org.mockito.exceptions.verification.TooManyActualInvocations: 
  runtimeContext.isDisablePeerDiscovery();
  Wanted 1 time:
  -> at com.quorum.tessera.discovery.DiscoveryFactoryTest.defaultConstructor(DiscoveryFactoryTest.java:80)
  But was 2 times:
  -> at com.quorum.tessera.discovery.DiscoveryFactory.provider(DiscoveryFactory.java:31)
  -> at com.quorum.tessera.discovery.DiscoveryFactory.provider(DiscoveryFactory.java:31)
  

• Root cause:
  It because this class doesn't reset the mocked objects at the very beginning, though we did reset MockContextHolder after each test. If we called this class right after other classes that have mocked objects unrested, there will be flaky tests.

The org.mockito.exceptions.verification.TooManyActualInvocations is found by running iDFlakies. After fixing the issue, there's no flaky tests in DiscoveryFactoryTest.java

[codecov-io]

Codecov Report

Merging #1194 (a6876fc) into master (5150289) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1194   +/-   ##
=========================================
  Coverage     99.50%   99.50%           
  Complexity     3081     3081           
=========================================
  Files           470      470           
  Lines          9054     9054           
  Branches        480      480           
=========================================
  Hits           9009     9009           
  Misses            5        5           
  Partials         40       40           

Flag	Coverage Δ	Complexity Δ
unittests	99.50% <ø> (ø)	0.00 <ø> (ø)

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