2.x: Assert instead of print Undeliverable in some tests

[akarnokd]

Set plugin error tracking in some tests to avoid printing out UndeliverableExceptions (sometimes repeatedly in race tests) and assert the specific exception(s) instead.

[codecov]

Codecov Report

Merging #6205 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6205      +/-   ##
============================================
- Coverage     98.23%   98.22%   -0.01%     
  Complexity     6200     6200              
============================================
  Files           667      667              
  Lines         44888    44888              
  Branches       6218     6218              
============================================
- Hits          44094    44093       -1     
  Misses          252      252              
- Partials        542      543       +1

Impacted Files	Coverage Δ	Complexity Δ
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ex/internal/subscriptions/SubscriptionArbiter.java	94.91% <0%> (-2.55%)	46% <0%> (-1%)
...perators/mixed/ObservableConcatMapCompletable.java	97.74% <0%> (-2.26%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.44% <0%> (-2.13%)	11% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...internal/operators/flowable/FlowableFromArray.java	94.91% <0%> (-1.7%)	3% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
... and 24 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cc45975...b6237c8. Read the comment docs.