What you are talking about here is harassment not terrorism.
Yes a certain number of people will be frightened by a hint of a threat like this one but by and large the press will not even report an incident like this one unless the device is an actual bomb that failed to go off. (If it bleeds it leads) The only place this will be reported is the local news.
So with the exception of security bulletins that would be issued by HLS to LEOs this Dry Run will be unknown to the general populace and will have little general effect except to irritate people because of longer slower lines because of a higher level of security for a short period of time until the threat level is lowered.
True terrorism requires dead bodies on the 6 o’clock news instilling fear in the majority of the population.
While media dissemination of events helps the terrorist cause by spreading the terror, anything which causes a change in the way people do things is effective.
Something under is bus is no biggie, but even the most downplaying article plants the thought in the minds of bus travellers that the bus might have a bomb underneath.
For nay bus company, that means failure to assuage the doubts of their passengers translates to decreased revenue, with the inevitable effect that now a great show will be made of checking under the bus.
Small, subtle perhaps, but effective in that resources have been tied up in hiring people to conspicuously wheel around mirrors to check for bombs so the passengers are happy.
While the driver should be fully capable, someone in a uniform wearing a badge would be more 'convincing' in the public eye, so resources are allocated to alleviate the presumed threat.
Any act which instills fear or causes a behavioural change in the target population is effective. While blood in the streets and collapsing buildings might be very effective, they also cause a reaffirmation of national resolve in Americans, a 'get the SOBs who did this' mentality, rather than the more stressful feeling of being less than secure in one's home country, a feeling of security which most Americans take for granted.
In this instance, "it could have been a bomb" is likely more effective psychologically than images of shattered (even children's) bodies on the TV. While the latter would cause grief in many, it would cause anger in many more, while the former causes ongoing doubt.