fix CHECK_THROWS_AS usage

"Note that the exception type is extended with const& and you should not include it yourself."

according to https://github.com/catchorg/Catch2/blob/devel/docs/assertions.md#exceptions

tests/context.cpp

@@ -75,10 +75,10 @@ TEST_CASE("context set/get options", "[context]")
 
     CHECK_THROWS_AS(
         context.set(static_cast<zmq::ctxopt>(-42), 5),
-        const zmq::error_t &);
+        zmq::error_t);
 
     CHECK_THROWS_AS(
         context.get(static_cast<zmq::ctxopt>(-42)),
-        const zmq::error_t &);
+        zmq::error_t);
 }
 #endif