Create an assertZeroAllocs test helper

[MrAlias]

func assertZeroAllocs(t *testing.T, f func(), msgAndArgs ...interface{}) {
	t.Helper()
	assert.Equal(t, 0.0, testing.AllocsPerRun(5, f, msgAndArgs...)
}

I suggest doing it as a separate PR so that more people also review this refactoring (and merge this PR as it is).

Originally posted by @pellared in #4957 (comment)

[pellared]

Closing per #4965 (review)