Unit test update to reflect bug found in interview

* Tony brought up the case where doing limited infection with a
  connected component much smaller than the target population size.
  Currently we just return the maximum connected component. The right
  thing to do is keep selecting new students at random to infect and
  walking from them, until we've filled our quota.

  This commit updates the unit tests to reflect this desired behavior.
  It does not change the implementation.

  TODO: to change the implementation, limited_infection will need a
  reference to our complete user population.