Fix compatibility with the upcoming version of testthat

[Enchufa2]

1 error  | 0 warnings | 0 notes.

    checking tests ... ERROR
    Running the tests in ‘tests/testthat.R’ failed.
    Last 13 lines of output:
      > test_check("simmer")
      1. Failure: the trajectory stores the right number of activities
(@test-trajectory.R#76)
      `output` does not match ".*(13
activities.*Seize.*nurse.*1.*Timeout.*function.*Release.*nurse.*1.*Branch.*1.*6
activities.*Seize.*doctor.*function.*Timeout.*function.*Release.*doctor.*function.*Branch.*1.*3
activities.*Seize.*administration.*1.*Timeout.*1.*Release.*administration.*1.*Rollback.*1.*Branch.*1.*Rollback.*1.*Rollback.*function.*SetAttribute.*1.*SetAttribute.*function).*".
      Actual value: ""


      Trajectory: anonymous, 1 activities
      testthat results
================================================================
      OK: 192 SKIPPED: 0 FAILED: 1
      1. Failure: the trajectory stores the right number of activities
(@test-trajectory.R#76)

      Error: testthat unit tests failed
      Execution halted

There are two common classes of
new failures caused by this version of testthat:

1. expect_output() no longer automatically prints the object. You'll need to
   explicitly print() if you want to check the output of a print method.
2. expect_error() now only compares against the actual error message,
   not the prefix, e.g. "Error in foo(): ". You might need to change you
   match message to take this into account.

It seems the problem falls into category 1.