Make useRealTimers play well with timers: fake

[thymikee]

Summary

We unnecessarily call useFakeTimers on every beforeEach, even though it's only needed when combined with resetMocks.
Fixes #3815

Test plan

Added an integration test.

[codecov-io]

Codecov Report

Merging #3858 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3858      +/-   ##
==========================================
- Coverage   57.63%   57.62%   -0.02%     
==========================================
  Files         194      194              
  Lines        6782     6784       +2     
  Branches        6        6              
==========================================
  Hits         3909     3909              
- Misses       2870     2872       +2     
  Partials        3        3

Impacted Files	Coverage Δ
packages/jest-jasmine2/src/index.js	0% <0%> (ø)	⬆️
packages/jest-cli/src/TestRunner.js	25.3% <0%> (ø)	⬆️
packages/jest-cli/src/reporters/SummaryReporter.js	22.58% <0%> (ø)	⬆️

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[rogeliog]

Does it also fix #3814?

[thymikee]

Haven't tried, but it seems like a node 7.4 issue. It works on every other version

[github-actions]

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