test: Introduce MockableSteadyClock::mock_time_point and ElapseSteady helper

This refactor clarifies that the MockableSteadyClock::mock_time_point
has millisecond precision by defining a type an using it.

Moreover, a ElapseSteady helper is added which can be re-used easily.

src/test/util/CMakeLists.txt

@@ -15,6 +15,7 @@ add_library(test_util STATIC EXCLUDE_FROM_ALL
   script.cpp
   setup_common.cpp
   str.cpp
+  time.cpp
   transaction_utils.cpp
   txmempool.cpp
   validation.cpp

src/test/util/time.cpp

@@ -0,0 +1,5 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <test/util/time.h>

src/test/util/time.h

@@ -0,0 +1,23 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_TEST_UTIL_TIME_H
+#define BITCOIN_TEST_UTIL_TIME_H
+
+#include <util/time.h>
+
+struct ElapseSteady {
+    MockableSteadyClock::mock_time_point::duration t{MockableSteadyClock::INITIAL_MOCK_TIME};
+    ElapseSteady()
+    {
+        (*this)(0s); // init
+    }
+    void operator()(std::chrono::milliseconds d)
+    {
+        t += d;
+        MockableSteadyClock::SetMockTime(t);
+    }
+};
+
+#endif // BITCOIN_TEST_UTIL_TIME_H