test: Make blockencodings_tests deterministic

refactor: CBlockHeaderAndShortTxIDs constructor now always takes an explicit nonce.
test: Make blockencodings_tests deterministic using fixed seed providing deterministic
CBlockHeaderAndShortTxID nonces and dummy transaction IDs.

Fixes very rare flaky test failures, where the ShortIDs of test transactions collide, leading to
`READ_STATUS_FAILED` from PartiallyDownloadedBlock::InitData and/or `IsTxAvailable` giving `false`
when the transaction should actually be available.

 * Use a new `FastRandomContext` with a fixed seed in each test, to ensure 'random' uint256s
   used as fake prevouts are deterministic, so in-turn test txids and short IDs are deterministic
   and don't collide causing very rare but flaky test failures.
 * Add new test-only/internal initializer for `CBlockHeaderAndShortTxIDs` that takes a specified
   nonce to further ensure determinism and avoid rare but undesireable short ID collisions.
   In a test context this nonce is set to a fixed known-good value. Normally it is random, as
   previously.

Flaky test failures can be reproduced with:

```patch
diff --git a/src/blockencodings.cpp b/src/blockencodings.cpp
index 695e8d806a..64d635a97a 100644
--- a/src/blockencodings.cpp
+++ b/src/blockencodings.cpp
@@ -44,7 +44,8 @@ void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {

 uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const {
     static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
-    return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+    // return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+    return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0x0f;
 }

```

to increase the likelihood of a short ID collision; and running

```shell
set -e;
n=0;
while (( n++ < 5000 )); do
    src/test/test_bitcoin --run_test=blockencodings_tests;
done
```

src/net_processing.cpp

@@ -2151,7 +2151,7 @@ void PeerManagerImpl::BlockDisconnected(const std::shared_ptr<const CBlock> &blo
  */
 void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& pblock)
 {
-    auto pcmpctblock = std::make_shared<const CBlockHeaderAndShortTxIDs>(*pblock);
+    auto pcmpctblock = std::make_shared<const CBlockHeaderAndShortTxIDs>(*pblock, GetRand<uint64_t>());
 
     LOCK(cs_main);
 
@@ -2549,7 +2549,7 @@ void PeerManagerImpl::ProcessGetBlockData(CNode& pfrom, Peer& peer, const CInv&
                 if (a_recent_compact_block && a_recent_compact_block->header.GetHash() == pindex->GetBlockHash()) {
                     MakeAndPushMessage(pfrom, NetMsgType::CMPCTBLOCK, *a_recent_compact_block);
                 } else {
-                    CBlockHeaderAndShortTxIDs cmpctblock{*pblock};
+                    CBlockHeaderAndShortTxIDs cmpctblock{*pblock, GetRand<uint64_t>()};
                     MakeAndPushMessage(pfrom, NetMsgType::CMPCTBLOCK, cmpctblock);
                 }
             } else {
@@ -6033,7 +6033,7 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
                         CBlock block;
                         const bool ret{m_chainman.m_blockman.ReadBlockFromDisk(block, *pBestIndex)};
                         assert(ret);
-                        CBlockHeaderAndShortTxIDs cmpctblock{block};
+                        CBlockHeaderAndShortTxIDs cmpctblock{block, GetRand<uint64_t>()};
                         MakeAndPushMessage(*pto, NetMsgType::CMPCTBLOCK, cmpctblock);
                     }
                     state.pindexBestHeaderSent = pBestIndex;