test, bench: make prevector and checkqueue swap member functions noexcept

Reason:
A swap must not fail; when a class has a swap member function, it should be declared noexcept.
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c84-a-swap-function-must-not-fail

src/test/fuzz/checkqueue.cpp

@@ -26,7 +26,7 @@ struct DumbCheck {
         return result;
     }
 
-    void swap(DumbCheck& x)
+    void swap(DumbCheck& x) noexcept
     {
     }
 };

src/test/fuzz/prevector.cpp

@@ -161,7 +161,7 @@ public:
         pre_vector.shrink_to_fit();
     }
 
-    void swap()
+    void swap() noexcept
     {
         real_vector.swap(real_vector_alt);
         pre_vector.swap(pre_vector_alt);