atf-c++ shouldn't catch exceptions from test cases

[wca]

atf-c++/tests.cpp:wrap_body() catches exceptions from the test case body. Assuming that the test framework executes each test in a child process, this should not be necessary. In fact, if any exception is thrown, the easiest way to debug what went wrong is if there is a crash dump. But the try..catch statements in this method preclude that from ever occurring.