Support ability to hide verdict of some testcases during contests

[jonathanirvings]

Add a feature to not give the verdict of all testcases during a contest.

In an onsite contest, compared to having a frozen scoreboard in the last several minutes of the contest (like ICPC WF), it is more interesting and suspenseful [1] to not have a full feedback during the contest, and instead only have a feedback for a small subset of testcases (e.g., smaller dataset like GCJ, or a large number of sample testcases like FBHC and TCO). This is especially true when all of the contestants know each other.

Using the latter format, each contestant must be confident and test their solution well. It is also more difficult to reverse-engineer the feedback and solve a task using wrong solutions. Each contestant also does not aware of their own score after the contest ends, adding elements of surprises and drama to the scoreboard reveal.

[1] Personal experience from attending several GCJ and ICPC Finals :)