quiet DRD error reports

testget

Complaint is of race between wait and Context cleanup.
shared_ptr<info_t> probably makes this safe, but
no down-side to explicit shutdown.

test/testput.cpp

@@ -493,7 +493,7 @@ void testErrorManual(ErrorSource::phase_t phase)
 
 struct CancelSource : public server::Source {
     const Value proto;
-    bool cancelled = false;
+    std::atomic<bool> cancelled{false};
 
     explicit CancelSource()
         :proto(nt::NTScalar{}.create())