Automatically enable MockitoExtension in JUnit 5 tests.

[anuraaga]

We commonly use mockito in tests so it should make sense to always have it enabled for junit5 tests. This change ensures it's always as easy (or easier) to write a junit5 test as junit4 in the codebase.

Migrated one test to demonstrate it works.

This extension defaults to mockito's strict stubs setting - @MockitoSettings can be applied to disable this. But Mockito seems to strongly recommend this and is going to default it in mockito 3 because it removes the needs to set stubs, then verify them - verification happens automatically with the strict stubs setting.

[codecov]

Codecov Report

Merging #1811 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1811      +/-   ##
============================================
- Coverage     73.24%   73.21%   -0.03%     
+ Complexity     8743     8739       -4     
============================================
  Files           776      776              
  Lines         34175    34175              
  Branches       4198     4198              
============================================
- Hits          25030    25020      -10     
- Misses         7011     7021      +10     
  Partials       2134     2134

Impacted Files	Coverage Δ	Complexity Δ
...meria/common/stream/RegularFixedStreamMessage.java	81.63% <0%> (-6.13%)	13% <0%> (-1%)
...lient/circuitbreaker/CircuitBreakerHttpClient.java	69.69% <0%> (-3.04%)	10% <0%> (-1%)
...com/linecorp/armeria/server/saml/SamlEndpoint.java	62.5% <0%> (-2.5%)	10% <0%> (-1%)
...om/linecorp/armeria/client/HttpSessionHandler.java	59.48% <0%> (-1.73%)	28% <0%> (-1%)
...inecorp/armeria/server/grpc/ArmeriaServerCall.java	86.16% <0%> (-1.59%)	81% <0%> (-1%)
...a/common/grpc/protocol/ArmeriaMessageDeframer.java	70.81% <0%> (-0.96%)	47% <0%> (-1%)
...inecorp/armeria/server/HttpResponseSubscriber.java	84.43% <0%> (+0.47%)	65% <0%> (+1%)	⬆️
.../linecorp/armeria/internal/IdleTimeoutHandler.java	100% <0%> (+20%)	5% <0%> (+1%)	⬆️

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[trustin]

Very nice. Learned a new trick :-)

[hyangtack]

Thanks!