test: refactor: Use FakeNodeClock over manual/global SetMockTime

Using SetMockTime in tests is problematic, because it often requires
verbose calls to
`SetMockTime(GetTime<std::chrono::seconds>() + offset)`.
Also, it requires manual `SetMockTime(0);` at the end.

Fix both issues by using FakeNodeClock.

src/test/httpserver_tests.cpp

@@ -805,7 +805,7 @@ BOOST_AUTO_TEST_CASE(http_server_socket_tests)
 {
     // Hard code the timestamp for the Date header in the HTTP response
     // Wed Dec 11 00:47:09 2024 UTC
-    SetMockTime(1733878029);
+    FakeNodeClock clock{1733878029s};
 
     // Prepare a request handler that just stores received requests so we can examine them.
     // Mutex is required to prevent a race between this test's main thread and the server's I/O loop.

src/test/mempool_fee_estimator_tests.cpp

@@ -105,6 +105,7 @@ BOOST_AUTO_TEST_CASE(calculate_max_weight_percentiles)
 
 BOOST_AUTO_TEST_CASE(mempool_fee_rate_estimator_cache)
 {
+    FakeNodeClock clock{};
     MemPoolFeeRateEstimatorCache cache;
     const uint256 tip_hash{uint256::ONE};
     const uint256 next_tip_hash{uint256{2}};
@@ -122,10 +123,9 @@ BOOST_AUTO_TEST_CASE(mempool_fee_rate_estimator_cache)
     BOOST_CHECK(cached->m_economical == economical);
     BOOST_CHECK(!cache.GetCachedEstimate(next_tip_hash));
 
-    SetMockTime(GetTime<std::chrono::seconds>() + CACHE_LIFE + std::chrono::seconds{1});
+    clock += CACHE_LIFE + std::chrono::seconds{1};
     BOOST_CHECK(cache.IsStale());
     BOOST_CHECK(!cache.GetCachedEstimate(tip_hash));
-    SetMockTime(0);
 }
 
 BOOST_AUTO_TEST_CASE(MempoolFeeRateEstimator)