test: Inline incorrect check in `util_tests` #35689

Assigning ToIntegral<int64_t>("-1") to the optional<uint64_t> n is a silent underflow. BOOST_CHECK_EQUAL then promotes int to uint64_t, which also underflows. The correct check is to do this inline. Part of clean up in #35587

<details> <summary>gdb</summary>

(gdb) next
886	    n = ToIntegral<int64_t>("-1", 16);
1: n = {<std::_Optional_base<unsigned long, true, true>> = {<std::_Optional_base_impl<unsigned long, std::_Optional_base<unsigned long, true, true> >> = {<No data fields>}, _M_payload = {<std::_Optional_payload_base<unsigned long>> = {_M_payload = {
          _M_empty = {<No data fields>}, _M_value = 18446744073709551615},
        _M_engaged = true}, <No data fields>}}, <std::_Enable_copy_move<true, true, true, true, std::optional<unsigned long> >> = {<No data fields>}, <No data fields>}
2: n = {<std::_Optional_base<unsigned long, true, true>> = {<std::_Optional_base_impl<unsigned long, std::_Optional_base<unsigned long, true, true> >> = {<No data fields>}, _M_payload = {<std::_Optional_payload_base<unsigned long>> = {_M_payload = {
          _M_empty = {<No data fields>}, _M_value = 18446744073709551615},
        _M_engaged = true}, <No data fields>}}, <std::_Enable_copy_move<true, true, true, true, std::optional<unsigned long> >> = {<No data fields>}, <No data fields>}
(gdb) next
887	    BOOST_CHECK_EQUAL(*n, -1);
1: n = {<std::_Optional_base<unsigned long, true, true>> = {<std::_Optional_base_impl<unsigned long, std::_Optional_base<unsigned long, true, true> >> = {<No data fields>}, _M_payload = {<std::_Optional_payload_base<unsigned long>> = {_M_payload = {
          _M_empty = {<No data fields>}, _M_value = 18446744073709551615},
        _M_engaged = true}, <No data fields>}}, <std::_Enable_copy_move<true, true, true, true, std::optional<unsigned long> >> = {<No data fields>}, <No data fields>}
2: n = {<std::_Optional_base<unsigned long, true, true>> = {<std::_Optional_base_impl<unsigned long, std::_Optional_base<unsigned long, true, true> >> = {<No data fields>}, _M_payload = {<std::_Optional_payload_base<unsigned long>> = {_M_payload = {
          _M_empty = {<No data fields>}, _M_value = 18446744073709551615},
        _M_engaged = true}, <No data fields>}}, <std::_Enable_copy_move<true, true, true, true, std::optional<unsigned long> >> = {<No data fields>}, <No data fields>}

</details>

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35689.

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Thx, nice change!

It should be simple to avoid any dirty state between test cases and avoid the std::optional::operator=() by just in-lining the test cases, so that the same (and other) issues does not happen again in the future.

but this lgtm.

ACK fa1cab673d4f5c6943249cfd5da01a025f113ff9

review ACK cd2a4bc5101ea28661a3fa3a70b6735b68ae70fc 🎬

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