Update TestHelper.java: trivial typo fix

[jnlopar]

Thank you for contributing to RxJava. Before pressing the "Create Pull Request" button, please consider the following points:

• Please give a description about what and why you are contributing, even if it's trivial.
  Just a small typo fix. I grepped for the same misspelling and it only appeared in this file.

• Please include the issue list number(s) or other PR numbers in the description if you are contributing in response to those.
  N/A

• Please include a reasonable set of unit tests if you contribute new code or change an existing one. If you contribute an operator, (if applicable) please make sure you have tests for working with an empty, just, range of values as well as an error source, with and/or without backpressure and see if unsubscription/cancellation propagates correctly.
  N/A

[codecov]

Codecov Report

Merging #6099 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6099      +/-   ##
============================================
- Coverage     98.25%   98.23%   -0.03%     
- Complexity     6194     6195       +1     
============================================
  Files           667      667              
  Lines         44862    44862              
  Branches       6213     6213              
============================================
- Hits          44081    44072       -9     
- Misses          239      241       +2     
- Partials        542      549       +7

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	89.55% <0%> (-4.48%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ex/internal/operators/flowable/FlowableCreate.java	96.12% <0%> (-1.62%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
... and 18 more

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