test: Add NodeClockContext

This makes it easier to use mock-time in tests. Also, it resets the
global mocktime, so that no state is leaked between test cases.

src/test/util/time.h

@@ -34,4 +34,32 @@ public:
     }
 };
 
+/// Helper to initialize the global NodeClock, let a duration elapse,
+/// and reset it after use in a test.
+class NodeClockContext
+{
+    NodeSeconds m_t{std::chrono::seconds::max()};
+
+public:
+    /// Initialize with the given time.
+    explicit NodeClockContext(NodeSeconds init_time) { set(init_time); }
+    explicit NodeClockContext(std::chrono::seconds init_time) { set(init_time); }
+    /// Initialize with current time, using the next tick to avoid going back by rounding to seconds.
+    explicit NodeClockContext() { set(++Now<NodeSeconds>().time_since_epoch()); }
+
+    /// Unset mocktime.
+    ~NodeClockContext() { set(0s); }
+
+    NodeClockContext(const NodeClockContext&) = delete;
+    NodeClockContext& operator=(const NodeClockContext&) = delete;
+
+    /// Set mocktime.
+    void set(NodeSeconds t) { SetMockTime(m_t = t); }
+    void set(std::chrono::seconds t) { set(NodeSeconds{t}); }
+
+    /// Change mocktime by the given duration delta.
+    void operator+=(std::chrono::seconds d) { set(m_t += d); }
+    void operator-=(std::chrono::seconds d) { set(m_t -= d); }
+};
+
 #endif // BITCOIN_TEST_UTIL_TIME_H