tests: remove assert_nothing_raised (part 2)

tests: remove assert_nothing_raised (part 2)

assert_nothing_raised ends up hiding errors and backtraces,
making things harder to debug.  Since Test::Unit already
fails on uncaught exceptions, there is no need to assert
on the lack of exceptions for a successful test run.

This is a followup to commit 5acf5522295c947d3118926d1a1077007f615de9