Make even deprecated test cases use the tc_try function (so that
all test cases look the same). Also make the skip reason for those
test cases be slightly more readable.

Make use of the tc_try function(s) in all "active" test cases.

Add a utility function, tc_try/2,3, for running test cases
in a uniform way.

The (second) notify test case had a faulty "condition" test
(which skipped the case in all cases except when it was
supposed to be skipped :)
Also, the test start and stop of the manager during the
test case has been adjusted. We try to keep track of how
long time each start and stop takes. And also, if they takes
to long, we skip the case.

Make stopping the manager (the entire manager sup tree)
more easy. The function(s) for starting and stopping the
manager that exist in the snmpm module is (mostly) intended
for testing, making the stop function more "potent" is no
real problem (now any process not just the parent) can stop
the manager process tree.

We have seen one case when it takes unreasonably long time
to start the manager, which causes the test case (notify_started01)
to fail. We try to monitor the start, and if it takes to long
a time, we instead skip.