Fix InitWithReset - rely on previously updated flags

[farazdagi]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• Normally in TestMain function is used to setup required features for package tests, e.g.:

func TestMain(m *testing.M) {
	resetCfg := featureconfig.InitWithReset(&featureconfig.Flags{
		EnablePeerScorer: true,
	})
	
	code := m.Run()
	// os.Exit will prevent defer from being called, so calling it directly.
	resetCfg()
	os.Exit(code)
}

• Then in individual test cases you may want to update that configuration (for example, I wanted to turn EnablePeerScorer off for just one case):

func TestBlocksFetcher_filterPeers(t *testing.T) {
	resetCfg := featureconfig.InitWithReset(&featureconfig.Flags{
		EnablePeerScorer: false,
	})
	defer resetCfg()
        // ... skip
}

• Now, the problem is that our current implementation of InitWithReset() will reset to &Flags{} (new empty flags object, with EnablePeerScorer assuming default value of false), instead of the one that is configured by the TestMain() (where EnablePeerScorer is set to true for the test suite).

Which issues(s) does this PR fix?

Other notes for review

[codecov]

Codecov Report

Merging #7598 into master will not change coverage.
The diff coverage is n/a.

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##           master    #7598   +/-   ##
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  Coverage   61.69%   61.69%           
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  Files         422      422           
  Lines       29712    29712           
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  Hits        18332    18332           
  Misses       8440     8440           
  Partials     2940     2940