mockImplemetation() resets the returnValue flag, but not the actual value

[aaronabramov]

fixes #3928

[codecov-io]

Codecov Report

Merging #3933 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3933      +/-   ##
=========================================
+ Coverage    58.1%   58.1%   +<.01%     
=========================================
  Files         196     196              
  Lines        6752    6753       +1     
  Branches        6       6              
=========================================
+ Hits         3923    3924       +1     
  Misses       2826    2826              
  Partials        3       3

Impacted Files	Coverage Δ
packages/jest-mock/src/index.js	91.92% <100%> (+0.03%)	⬆️

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[github-actions]

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