test: refactor: Make unsigned promotion explicit

Integer promotion will already turn the `signed` into `unsigned` in
those lines. However, make the `unsigned` explicit so that the code is
clearer and a compiler warning is avoided when switching to m_rng:

| test/validation_block_tests.cpp: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
|   136 |     bool gen_invalid = m_rng.randrange(100) < invalid_rate;
|       |                        ~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
|   137 |     bool gen_fork = m_rng.randrange(100) < branch_rate;
|       |                     ~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~
| 2 warnings generated.

Author: MarcoFalke

src/test/validation_block_tests.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2018-2022 The Bitcoin Core developers
+// Copyright (c) 2018-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -133,8 +133,8 @@ void MinerTestingSetup::BuildChain(const uint256& root, int height, const unsign
 {
     if (height <= 0 || blocks.size() >= max_size) return;
 
-    bool gen_invalid = InsecureRandRange(100) < invalid_rate;
-    bool gen_fork = InsecureRandRange(100) < branch_rate;
+    bool gen_invalid = m_rng.randrange(100U) < invalid_rate;
+    bool gen_fork = m_rng.randrange(100U) < branch_rate;
 
     const std::shared_ptr<const CBlock> pblock = gen_invalid ? BadBlock(root) : GoodBlock(root);
     blocks.push_back(pblock);