scripted-diff: Rename SteadyClockContext to FakeSteadyClock

SteadyClockContext and FakeNodeClock are both LimitOne RAII helpers that mock a
clock in tests -- the steady clock and the node clock, respectively. Rename the
former so the two follow a consistent FakeXClock naming scheme.

-BEGIN VERIFY SCRIPT-
sed -i 's/SteadyClockContext/FakeSteadyClock/g' $(git grep -l SteadyClockContext)
-END VERIFY SCRIPT-

src/test/pcp_tests.cpp

@@ -222,7 +222,7 @@ private:
     const std::vector<TestOp> m_script;
     mutable size_t m_script_ptr = 0;
     mutable std::chrono::milliseconds m_time_left;
-    mutable SteadyClockContext m_clock{};
+    mutable FakeSteadyClock m_clock{};
     mutable bool m_connected{false};
     mutable CService m_bound;
     mutable CNetAddr m_local_ip;