test: fix serve errors on windows

[snitin315]

What kind of change does this PR introduce?
tests

Did you add tests for your changes?
Yes
If relevant, did you update the documentation?
no
Summary
Fix tests on master.

Follow up #3783

Does this PR introduce a breaking change?
No

Other information
No

[snitin315]

We don't test on the exit code for the serve because we exit forcefully and the exit code is different on Linux/windows.

webpack-cli/test/utils/test-utils.js

Lines 31 to 37 in eaca5d2

	const processKill = (process) => {
	if (isWindows) {
	exec("taskkill /pid " + process.pid + " /T /F");
	} else {
	process.kill();
	}
	};

[codecov]

Codecov Report

Merging #3784 (1806e89) into master (eaca5d2) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3784   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files          22       22           
  Lines        1624     1624           
  Branches      462      462           
=======================================
  Hits         1488     1488           
  Misses        136      136           

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