…ent-vector errors

11f3bc2 refactor: Reserve vectors in fuzz tests (Lőrinc)
152fefe refactor: Preallocate PrevectorFillVector(In)Direct without vector resize (Lőrinc)
a774c7a refactor: Fix remaining clang-tidy performance-inefficient-vector errors (Lőrinc)

Pull request description:

  PR inspired by #29608 (comment) (and #29458, #29606, #29607, #30093).

  The `clang-tidy` check can be run via:
  ```bash
  cmake -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DBUILD_BENCH=ON -DBUILD_FUZZ_BINARY=ON -DBUILD_FOR_FUZZING=ON && cmake --build build -j$(nproc)

  run-clang-tidy -quiet -p build -j $(nproc) -checks='-*,performance-inefficient-vector-operation' | grep -v 'clang-tidy'
  ```
  which revealed 3 tests and 1 prod warning (+ fuzz and benching, found by hebasto).
  Even though the tests aren't performance critical, getting rid of these warnings (for which the checks were already enabled via https://github.com/bitcoin/bitcoin/blob/master/src/.clang-tidy#L18, see below), the fix was quite simple.

  <details>
  <summary>clang-tidy -list-checks</summary>

  ```bash
  cd src && clang-tidy -list-checks | grep 'vector'
      performance-inefficient-vector-operation
  ```

  </details>

  <details>
  <summary>Output before the change</summary>

  ```
  src/test/rpc_tests.cpp:434:9: error: 'emplace_back' is called inside a loop; consider pre-allocating the container capacity before the loop [performance-inefficient-vector-operation,-warnings-as-errors]
    433 |     for (int64_t i = 0; i < 100; i++) {
    434 |         feerates.emplace_back(1 ,1);
        |         ^

  src/test/checkqueue_tests.cpp:366:13: error: 'emplace_back' is called inside a loop; consider pre-allocating the container capacity before the loop [performance-inefficient-vector-operation,-warnings-as-errors]
    365 |         for (size_t i = 0; i < 3; ++i) {
    366 |             tg.emplace_back(
        |             ^

  src/test/cuckoocache_tests.cpp:231:9: error: 'emplace_back' is called inside a loop; consider pre-allocating the container capacity before the loop [performance-inefficient-vector-operation,-warnings-as-errors]
    228 |     for (uint32_t x = 0; x < 3; ++x)
    229 |         /** Each thread is emplaced with x copy-by-value
    230 |         */
    231 |         threads.emplace_back([&, x] {
        |         ^

  src/rpc/output_script.cpp:127:17: error: 'push_back' is called inside a loop; consider pre-allocating the container capacity before the loop [performance-inefficient-vector-operation,-warnings-as-errors]
    126 |             for (unsigned int i = 0; i < keys.size(); ++i) {
    127 |                 pubkeys.push_back(HexToPubKey(keys[i].get_str()));
        |                 ^
  ```

  And the fuzz and benchmarks, noticed by hebasto: #31305 (comment)

  </details>

ACKs for top commit:
  maflcko:
    review ACK 11f3bc2 🎦
  achow101:
    ACK 11f3bc2
  theuni:
    ACK 11f3bc2
  hebasto:
    ACK 11f3bc2, tested with clang 19.1.5 + clang-tidy.

Tree-SHA512: 41691c19f35c63b922a95407617a54f9bff1af3f95f99d15642064f321df038aeb1ae5f061f854ed913f69036807cc28fa6222b2ff4c24ef43b909027fa0f9b3