This quote seems to apply to John Proctor, Elizabeth Proctor, Giles Corey, Reverend Hale and Abigail.
Abigail is the person challenged initially in the play. She has been caught dancing naked in the woods, drinking blood, etc. The penalty for such behavior is death. She has no parents to help, guide or protect her and she is already a dubious personality in Salem. Conflict finds her right away in the story of the play. Her response: deflect blame by pointing the finger at others.
In reponse to the conflict set against her, Abigail shows herself to be savvy, smart, manipulative and, utlimately, a coward.
I agree that everyone in town is tested in some fashion by the events of the Witch Trials. Rev. Hale's faith is tested as he is forced to question the foundations of his beliefs in God, the devil, and the courts. The Putnams are tested as they are presented with the opportunity to be selfless or selfish--and we know which they chose. The Proctors' marriage is tested and it is found to be strong; the courts and judges are tested and found to be flawed; and the town is tested and found wanting as it allows these atrocities to take place.
The above answers make sense, but I would point out that it is not only the "good" people who are tested and proven. The conflict in this book proves the worst about some people. Abigail is one example of someone who is proven to be selfish and power-hungry. So, we should also note that many people in the play are tested and proven to be weak or greedy or power hungry.
I am most impressed by the fact that John Proctor is a reluctant hero. It is obvious that he wishes to avoid confronting the members of the witch hunt, yet each time he is forced to make a decision ... to turn his back on his neighbors who have been convicted, or to stand with them, John Proctor reluctantly does the "right thing." As he goes to the gallows, his wife says, "He has his goodness now; God forbid that I take it from him." In other words, finally John Proctor's goodness has emerged for he has been "tested and proven" in the conflict attending his conviction.
After a cursory Internet search, it seems this statement is a very popular creative writing prompt. It asks the writer to consider a grave trial and explain how he/she would deal with such a serious situation. It is a very appropriate prompt for Arthur Miller's The Crucible. John Proctor is characterized as a weak man, morally and spiritually. He does not attend church as often as the leadership would like, and falls prey to his own lust for the servant girl in his home. In the first scene that features John and his wife Elizabeth, he seems to only wish to placate his wife, to whom he is still apologizing for his indiscretion.
However, as the witchcraft trials begin to spiral out of control, John shows his strength, trying to be a voice of reason. His bravery is punished, as he is accused of witchcraft as a result of his apparent undermining of the court. That he does not back down is our takeaway from the play: one does not have to be a political, military, or spiritual leader to be great. His downfall is a tragedy, but not a hollow one.