test: Drop ++ from NodeClockContext default constructor

The increment was originally added so that mocked time would not appear
to go backward relative to real-clock timestamps captured before
construction, since Now<NodeSeconds>() rounds the current time down to
a whole second. In practice the tests do not mix real and mocked
timestamps in a way that exposes this, so the increment is unnecessary.

src/test/util/time.h

@@ -44,8 +44,8 @@ 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()); }
+    /// Initialize with current time.
+    explicit NodeClockContext() { set(Now<NodeSeconds>()); }
 
     /// Unset mocktime.
     ~NodeClockContext() { set(0s); }