Use `assert_raise` and `assert_nothing_raised` in `mruby-sleep` tests

mrbgems/mruby-sleep/test/sleep_test.rb

-def run_with_catching_error &b
-  e = nil
-  begin
-    b.call
-  rescue => _e
-    e = _e
-  end
-
-  return e
-end
-
 assert("sleep works") do
-  e = run_with_catching_error { sleep 1 }
-
-  assert_nil e
+  assert_nothing_raised { sleep(1) }
 end
 
 assert("sleep would not accept negative value") do
-  e = run_with_catching_error{ sleep(-1) }
-
-  assert_not_equal e, nil
-  assert_equal e.class, ArgumentError
+  assert_raise(ArgumentError) { sleep(-1) }
 end
 
 assert("usleep works") do
-  e = run_with_catching_error { usleep 100 }
-
-  assert_nil e
+  assert_nothing_raised { usleep(100) }
 end
 
 assert("usleep would not accept negative value") do
-  e = run_with_catching_error{ usleep(-100) }
-
-  assert_not_equal e, nil
-  assert_equal e.class, ArgumentError
+  assert_raise(ArgumentError) { usleep(-100) }
 end