Merge #21689: test: Remove intermittently failing and not very meaningful `BOOST_CHECK` in `cnetaddr_basic`

63631beef6a0046390469971adf4500718ab34ad test: Remove intermittently failing and not very meaningful `BOOST_CHECK` in `cnetaddr_basic` (practicalswift)

Pull request description:

  Remove intermittently failing and not very meaningful `BOOST_CHECK` in `cnetaddr_basic`.

  Fixes #21682.

  Rationale from https://github.com/bitcoin/bitcoin/issues/21682#issuecomment-819897122:

  > I've looked at that test before and I don't think that specific `BOOST_CHECK` makes much sense TBH :)
  >
  > 1.) I don't understand why we test if `ToString()` output includes `%zone_index`: it clearly doesn't on some platforms, so we cannot rely on it anyways. Then why test it?
  >
  > 2.) And perhaps more fundamentally: why would we even _want_ to have `%zone_index` in our textual `ToString()` output? I think the expectation is to get say `fe80::1ff:fe23:4567:890a` (without zone index) and not say `fe80::1ff:fe23:4567:890a%eth2 ` or `fe80::1ff:fe23:4567:890a%3 `when doing `ipv6_addr.ToString()` :)

ACKs for top commit:
  MarcoFalke:
    review ACK 63631beef6a0046390469971adf4500718ab34ad

Tree-SHA512: 06863d1edfb9ad1ca9bcae09cf3f0f47b58bb29d222b70799c3dc059b96452889026e4b99b132782846d9896e3e798d17c7f9406e0e6a0bec1bffc6edb54e9df

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