2.x: Add public constructor for TestScheduler that takes the time.

[vanniktech]

• adds a time constructor to TestScheduler
• should I make it experimental or will it just be stable?

Fixes #5901

[akarnokd]

"Creates a new TestScheduler with the specified initial virtual time."

[akarnokd]

"Creates a new TestScheduler with initial virtual time of zero."

[codecov]

Codecov Report

Merging #5906 into 2.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5906      +/-   ##
============================================
- Coverage     98.02%   98.02%   -0.01%     
- Complexity     5993     5996       +3     
============================================
  Files           655      655              
  Lines         43924    43928       +4     
  Branches       6086     6086              
============================================
+ Hits          43056    43059       +3     
- Misses          256      266      +10     
+ Partials        612      603       -9

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/schedulers/TestScheduler.java	100% <100%> (ø)	11 <0> (ø)	⬇️
...ex/internal/operators/flowable/FlowableCreate.java	89.93% <0%> (-7.8%)	6% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-5.38%)	9% <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%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.39% <0%> (-1.74%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.93% <0%> (-1.07%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.48% <0%> (-0.63%)	50% <0%> (ø)
... and 22 more

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