Avoid mutating contexts when updating snapshots in watch mode

[thymikee]

Summary

Fixes #3330 and #3316.
Mutation of contexts.config object causes some unexpected behaviors, such as retaining updateSnapshots flag. Removing mutation resolves the issue, so that overrideConfig only overrides the config for following run.

Test plan

jest

[codecov-io]

Codecov Report

Merging #3353 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3353   +/-   ##
=======================================
  Coverage   64.93%   64.93%           
=======================================
  Files         176      176           
  Lines        6514     6514           
  Branches        4        4           
=======================================
  Hits         4230     4230           
  Misses       2283     2283           
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-cli/src/watch.js	74.19% <100%> (ø)	⬆️

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[cpojer]

I have a PR coming in which will make this obsolete. Sorry about being slow.

[github-actions]

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