Merge #20584: Declare de facto const reference variables/member functions as const

31b136e580 Don't declare de facto const reference variables as non-const (practicalswift)
1c65c075ee Don't declare de facto const member functions as non-const (practicalswift)

Pull request description:

  _Meta: This is the second and final part of the `const` refactoring series (part one: #20581). **I promise: no more refactoring PRs from me in a while! :)** I'll now go back to focusing on fuzzing/hardening!_

  Changes in this PR:
  * Don't declare de facto const member functions as non-const
  * Don't declare de facto const reference variables as non-const

  Awards for finding candidates for the above changes go to:
  * `clang-tidy`'s [`readability-make-member-function-const`](https://clang.llvm.org/extra/clang-tidy/checks/readability-make-member-function-const.html)  check ([list of `clang-tidy` checks](https://clang.llvm.org/extra/clang-tidy/checks/list.html))
  * `cppcheck`'s `constVariable` check ([list of `cppcheck` checks](https://sourceforge.net/p/cppcheck/wiki/ListOfChecks/))

  See #18920 for instructions on how to analyse Bitcoin Core using Clang Static Analysis, `clang-tidy` and `cppcheck`.

ACKs for top commit:
  ajtowns:
    ACK 31b136e580
  jonatack:
    ACK 31b136e580
  theStack:
    ACK 31b136e580 ❄️

Tree-SHA512: f58f8f00744219426874379e9f3e9331132b9b48e954d24f3a85cbb858fdcc98009ed42ef7e7b4619ae8af9fc240a6d8bfc1c438db2e97b0ecd722a80dcfeffe

src/test/util/setup_common.cpp

@@ -239,12 +239,12 @@ TestChain100Setup::~TestChain100Setup()
     gArgs.ForceSetArg("-segwitheight", "0");
 }
 
-CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CMutableTransaction& tx)
+CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CMutableTransaction& tx) const
 {
     return FromTx(MakeTransactionRef(tx));
 }
 
-CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CTransactionRef& tx)
+CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CTransactionRef& tx) const
 {
     return CTxMemPoolEntry(tx, nFee, nTime, nHeight,
                            spendsCoinbase, sigOpCost, lp);