…h unit tests

8c03318 consensus/doc: explain `GetValueOut()` precondition (Lőrinc)
82ef92c consensus/doc: explain unreachable `bad-txns-fee-outofrange` check (Lőrinc)
232a2bc consensus/test: add out-of-range output unit tests for `CTransaction::GetValueOut` (Lőrinc)
aa87aae consensus/test: add `MoneyRange` unit tests for `CheckTxInputs` (Lőrinc)

Pull request description:

  ### Problem
  Coverage reports indicate that a few consensus related validations aren't exercised in unit-, and some not even in the functional-tests:
  Inspired by the coverage reports:
  * ["bad-txns-premature-spend-of-coinbase"](https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/consensus/tx_verify.cpp.gcov.html#L180): Only covered in functional tests
  * ["bad-txns-inputvalues-outofrange"](https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/consensus/tx_verify.cpp.gcov.html#L187): Unreachable in functional tests [1], uncovered in unit tests
  * ["bad-txns-in-belowout"](https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/consensus/tx_verify.cpp.gcov.html#L193): Only covered in functional tests
  * ["GetValueOut: value out of range"](https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/primitives/transaction.cpp.gcov.html#L103) and [total coverage report](https://maflcko.github.io/b-c-cov/total.coverage/src/primitives/transaction.cpp.gcov.html#L103)

  Replacing them with explicit throws still passes all unit (and sometimes even functional) tests, confirming those branches are not being exercised, see: l0rinc#112

  ### Fixes

  Add minimal unit test coverage for `Consensus::CheckTxInputs` invalid outcomes for `bad-txns-premature-spend-of-coinbase`, `bad-txns-inputvalues-outofrange`, `bad-txns-in-belowout`.
  Add a unit test covering `CTransaction::GetValueOut()` throwing for out of range values.
  After the prerequisits are tested, document why `bad-txns-fee-outofrange` is unreachable - while keeping the check in place because it is consensus-critical code.

ACKs for top commit:
  maflcko:
     lgtm ACK 8c03318 🍴
  darosior:
    utACK 8c03318
  sedited:
    ACK 8c03318

Tree-SHA512: 91c65dda99b42d12de99c58b02df0f861203f97d268329a3ecce79bd681fcaf847f508c1d9f2256b2b92a953a94d868cbae647f378def92484681d771722ea27