2.x: add TestSubscriber.withTag

[akarnokd]

This PR adds the method withTag to the TestBaseConsumer that allows setting a textual tag which is then appended to the assertion failure's message:

Failure message (latch = 1, values = 0, errors = 0, completions = 0, tag = x: 5, y: 7)

Use case, for example, is to add the parameters of the current flow to help identify what settings caused the failure. This comes up with unit tests that have some form of (nested) loop(s) to verify multiple parameter ranges. (It is often much easier than trying to work out JUnit rules and such.)

[codecov]

Codecov Report

Merging #5137 into 2.x will decrease coverage by -0.11%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5137      +/-   ##
============================================
- Coverage     95.96%   95.86%   -0.11%     
+ Complexity     5611     5600      -11     
============================================
  Files           620      620              
  Lines         39820    39826       +6     
  Branches       5599     5600       +1     
============================================
- Hits          38215    38180      -35     
- Misses          633      662      +29     
- Partials        972      984      +12

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/reactivex/observers/BaseTestConsumer.java	98.04% <100%> (+0.05%)	93 <1> (+2)	✅
...rnal/subscribers/SinglePostCompleteSubscriber.java	87.17% <ø> (-12.83%)	13% <ø> (-2%)
...ternal/operators/flowable/FlowableSampleTimed.java	88.23% <ø> (-8.83%)	3% <ø> (ø)
...a/io/reactivex/processors/SerializedProcessor.java	89.36% <ø> (-8.52%)	26% <ø> (-2%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <ø> (-6.13%)	2% <ø> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	84.97% <ø> (-5.7%)	56% <ø> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	90.9% <ø> (-5.46%)	2% <ø> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <ø> (-3.64%)	9% <ø> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <ø> (-3.28%)	2% <ø> (ø)
... and 44 more

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[vanniktech]

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