fix(mocks): fix conflict between mocks and skip

[nfischer]

This fixes a conflict between mocks.init() and
utils.skipOnWin/skipOnUnix. mocks.init() mocks out process.stderr.write,
which utils.js implicitly depends on.

Instead, preserve stderr.write in a local variable to avoid polluting
mocked stdio and to correctly output warning messages.

Fixes #862
Test: locally apply mocks.init() inside test/which.js

[codecov-io]

Codecov Report

Merging #863 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #863   +/-   ##
======================================
  Coverage    95.8%   95.8%           
======================================
  Files          34      34           
  Lines        1262    1262           
======================================
  Hits         1209    1209           
  Misses         53      53

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[freitagbr]

LGTM