2.x: make withLatestFrom conditional subscriber, test cold consumption

[akarnokd]

This PR adjusts #5494 by converting the withLatestFrom operators into conditional subscribers and adds unit tests that verify a cold source is consumed fully if there is no event(s) from the other source(s).

(Also there was a missing space before the bracked and the new unit test name had the test prefix.)

[codecov]

Codecov Report

Merging #5495 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5495      +/-   ##
============================================
- Coverage     96.06%   96.03%   -0.03%     
+ Complexity     5807     5800       -7     
============================================
  Files           631      631              
  Lines         41299    41303       +4     
  Branches       5743     5745       +2     
============================================
- Hits          39674    39666       -8     
- Misses          639      649      +10     
- Partials        986      988       +2

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableWithLatestFrom.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...operators/flowable/FlowableWithLatestFromMany.java	99.24% <100%> (+1.55%)	7 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-5.99%)	2% <0%> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...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%)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-3.51%)	9% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-2.84%)	31% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
... and 43 more

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