refactor(tracex): do not depend on strings for event names (#777)

Rather than matching a string, match a type.

This is more robust considering future refactorings.

We're confident the names did not change in _this_ refactoring
because we're still testing the same strings in the tests.

Part of https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/saver_test.go

@@ -12,7 +12,7 @@ func TestSaver(t *testing.T) {
 	wg.Add(parallel)
 	for idx := 0; idx < parallel; idx++ {
 		go func() {
-			saver.Write(Event{})
+			saver.Write(&EventReadFromOperation{&EventValue{}})
 			wg.Done()
 		}()
 	}