test: Fix MSVC warning C4101 "unreferenced local variable"

src/univalue/test/object.cpp

@@ -20,7 +20,7 @@
         try { \
             (stmt); \
             assert(0 && "No exception caught"); \
-        } catch (excMatch & e) { \
+        } catch (excMatch&) { \
         } catch (...) { \
             assert(0 && "Wrong exception caught"); \
         } \