[SPARK-23435][2.4][SPARKR][TESTS] Update testthat to >= 2.0.0#27379

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HyukjinKwon wants to merge 1 commit intoapache:branch-2.4from

HyukjinKwon:testthat-2.0

Commits on Jan 29, 2020

[SPARK-23435][SPARKR][TESTS][2.4] Update testthat to >= 2.0.0

zero323
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HyukjinKwon
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