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/blockencodings.cpp

@@ -17,8 +17,8 @@
 
 #include <unordered_map>
 
-CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block) :
-        nonce(GetRand<uint64_t>()),
+CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce) :
+        nonce(nonce),
         shorttxids(block.vtx.size() - 1), prefilledtxn(1), header(block) {
     FillShortTxIDSelector();
     //TODO: Use our mempool prior to block acceptance to predictively fill more than just the coinbase