Check whether we should output to stdout or stderr

[mjesun]

This PR is related to another two changes deployed this morning: #3945 and #3946. These two had to be rolled back because they were causing issues in CI, however they were working locally.

The cause of the breakage in CI was due to the fact that the is-ci module was not being mocked. Since CI sessions are considered non-interactive, the reporter was not outputting anything and the test failed. After adding a mock to the module (jest.mock('is-ci', () => false)) tests are passing now.

Code for the reporter itself has not been changed when comparing it with the two earlier PRs, so the release done in-between (jest@20.1-alpha.3) is safe in that sense.

[codecov-io]

Codecov Report

Merging #3953 into master will increase coverage by 1.8%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #3953     +/-   ##
=========================================
+ Coverage   58.06%   59.86%   +1.8%     
=========================================
  Files         196      196             
  Lines        6758     6763      +5     
  Branches        6        6             
=========================================
+ Hits         3924     4049    +125     
+ Misses       2831     2711    -120     
  Partials        3        3

Impacted Files	Coverage Δ
...ackages/jest-cli/src/reporters/default_reporter.js	91.04% <100%> (+86.2%)	⬆️
packages/jest-cli/src/reporters/utils.js	62.5% <0%> (+2.08%)	⬆️
packages/jest-cli/src/reporters/base_reporter.js	80% <0%> (+20%)	⬆️
...ckages/jest-cli/src/reporters/get_result_header.js	78.57% <0%> (+50%)	⬆️
packages/jest-cli/src/reporters/status.js	93.75% <0%> (+89.06%)	⬆️

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[github-actions]

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